World 12, Return to Doorhaven, presents itself as a second version of World 1, and its new gimmicks, Infinite Keys and Gates, can be thought of as alternate versions of the basic Keys and Doors that World 1 introduces... so what if the other ten Part 1 worlds also had "return" worlds, each with alternate versions of the gimmicks that world introduces? I've thought of a list of ideas for these "return" worlds, which I'll label as Worlds R2, R3, and so on (meaning Return to Doorhaven would be World R1 instead of World 12). A couple of these ideas are returning from my previous ideas post.

World R2: Rainy Resort
This one takes place indoors - I imagine it has a bluish-purple color scheme - but the rain can still be seen through the windows.

I've thought of two ways to make a Master Key variation for Gates, so here's both of them:
Checkerboard Keys
Checkerboard Keys are the same shape as Master Keys, but with a white-and-black checkerboard pattern instead of straight gold. These, like Master Keys, have an activated use: when you touch a Gate while using a Checkerboard Key, then 1 Checkerboard Key is spent and that Gate becomes "solidified" and will no longer change state: it will remain closed if it was closed when solidified and it will remain open if it was open when solidified, regardless of any changes to your Key counts afterwards. Negative Checkerboard Keys un-solidify Gates that were already solidified, returning them to their normal behavior.
Using a positive imaginary Checkerboard Key on a Gate makes it so whatever its solidification status is normally, it'll be the opposite when you're in I-View: for example, if it was solidified with a real Checkerboard Key while closed but a positive imaginary Checkerboard Key is then used on it, then the Gate remains closed on real View but has its normal behavior when in I-View. A negative imaginary Checkerboard Key undoes this offset, making the I-View behavior to match the regular View behavior again.
Of course, you can't use a positive real Checkerboard Key on a Gate that's already solidified, a negative real Checkerboard Key on a Gate that's already not solidified, a positive imaginary Checkerboard Key on a Gate that's already I-View offset, or a negative imaginary Checkerboard Key on a Gate that's already not I-View offset.
Gates with Checkerboard or Pure on them are immune to Checkerboard Keys.

Transparent Keys
These keys are semi-transparent white, fading between more and less transparency. If you're using a Transparent Key when you touch a closed gate, the gate will become temporarily open, remaining open until you stop touching it, then doing an animation to give you a second or two to go back to it if you didn't intend to leave it yet (that timer will, of course, be reset if you do start touching it again during this time), then going back to its normal behavior.
Negative Transparent Keys turn open gates closed until you stop touching them, which may be used in a platforming-focused level.
I can't think of a good twist on these to use for imaginary ones, so I guess imaginary Transparent Keys either do the same thing as their real counterparts or just don't do anything (like how imaginary amounts of Aura keys don't do anything).
Gates with Transparent or Pure on them are immune to Transparent Keys.

Blank Door variation:
Refresh Keys
These Keys can only be collected if you have exactly 0 of that Key color - if you don't, you'll pass right through the Key without collecting it. This is represented by horizontal lines moving from top to bottom of the Key. These lines are normally in a slightly dark version of the Key's color - if they're in a different color than the Key, then it's the color of the lines that you have to have 0 of to pick up that Key. For example, a 3 Purple Refresh Key with Cyan lines can only be picked up if you have no Cyan Keys.

World R3: Compartmentalized Infrastructure
A variant of The Infrastructure where the gears and cubes in the background are sectioned off into separate cubicles.

Exact Keys variation:
Exact Doors
Exact locks only open if you have exactly the right number of keys: an 4 Orange Exact lock can be opened if you have 4 Orange Keys, but not if you have 5 Orange Keys. These locks still spend the appropriate number of Keys, of course. However, Exact locks only care about the count in that dimension, so a 3 Blue Exact lock is okay with 3, 3+i, 3-3i, 3+7i, or any 3+bi amount of keys, as long as the real part is exactly 3. When it comes to Combo Doors, the Exact property is applied to individual locks rather than the door as a whole.

Blast Door variation:
Partial Blast Doors
A "1/2 Blast lock", represented as X/2, spends exactly half of your keys of that color - so if you open an Orange 1/2 Blast Door with 8 Orange Keys, it spends 4 of them. To ensure that the Key counts remain integers, a 1/2 Blast lock can only be opened if your count of that key color is a multiple of two. Blast locks for higher reciprocals (1/3, 1/4, 1/5, etc.) also exist, and of course each of the four "signs" has separate variants like for regular Blast Doors (for example, an imaginary 1/3 Blast lock is represented as +/3). Perhaps other fractions like 2/3 exist too, but maybe those are just achieved via Combo Doors with the reciprocals (like how a single lock can never be complex, only pure real or pure imaginary, and Combo Doors are used to make complex doors). In a scenario where the cost doesn't apply, like when looking at a Gate instead of a Door or when your Key count for that color is Starred, only the "must be the appropriate sign and must be a multiple of the denominator" rules are relevant, like how regular Blast Gates only care that you have some amount of the relevant sign of that Key color. Partial All Doors also exist (a 1/2 All Door would be represented as =/2).

World R4: Chilled Temple
It's wintertime at the Starlit Temple now. The background is whiter, the starlight sparkles are replaced with snow (though the snow is still drifting peacefully, unlike Frozen Palace), and ice lines the ground.

Auras variation:
Ice, Mud, and Ink Keys
These three key types are special: Ice Keys can open frozen doors, Mud Keys can open eroded doors, and Ink Keys can open painted doors. For example, if a 6 White Door is frozen, then while you can't open it normally, you can spend 6 Ice Keys to open it. When using Ice, Mud, and Ink Keys, all locks on that door are considered to be of the key type being used. If a door is both frozen and eroded, then it's treated as if you're opening both an Ice version of the door and a Mud version of the door at once, and only if both of those will work does it open.
There are, of course, regular locks and doors for Ice, Mud, and Ink. These can still be frozen/eroded/painted - a plain Ice Door requires Ice Keys, a painted Ice Door requires Ink Keys, and a frozen and painted Ice Door requires both Ice Keys and Ink Keys. An Ice Door that's just frozen is almost the same as a regular Ice Door, but not quite: it's immune to Master Keys because it's frozen, and while the Brown curse will still hit the door, that won't change the fact that you use Ice Keys to open it (because it's frozen).

These three key types might be enough for this world on their own, but if not, here's an add-on to make these gimmicks more versatile:
Dark Auras
Maroon, Forest, and Navy Keys are dark red, dark green, and dark blue keys respectively, and they all have auras. The Maroon aura will freeze any door you get close enough to, the Forest aura will erode any door you get close enough to, and the Navy aura will paint any door you get close enough to.
Here's how it's determined which auras you have: if your amount of real Blue Keys is positive and at least 3 greater than your amount of real Navy Keys, then you have the Blue aura. If your amount of real Navy Keys is positive and at least 3 more than your amount of real Blue Keys, then you have the Navy aura. Otherwise, you have neither of the two. Same goes for Green and Forest (with a difference threshold of 5), and for Red and Maroon (with a difference threshold of 1). Yes, this means you can have the Green aura with less than 5 Green Keys if your amount of Forest Keys is negative... but you still need at least 1 Green Key to have the Green aura. Imaginary amounts of these six keys have no bearing on the auras.
Red doors (and any door with Red on it, such as a Bicolored Door with a Red spending color or a Combo Door with at least one Red lock) are immune to the Maroon aura and vice versa, Green doors are immune to the Forest aura and vice versa, and Blue doors are immune to the Navy aura and vice versa. Notably, Pure is not immune to these "dark" auras, just as it doesn't protect from the Red, Green, and Blue auras.

There really doesn't need to be a variation to Master Doors, as these aura extensions are more than enough to make their own world... but here's a Master Door variation anyway:
Rainbow Keys
Rainbow Keys can be used in place of any Key color: for example, you can open a 6 Pink Door by spending 6 Rainbow Keys, you can open a 4 Red Door by spending 4 Rainbow Keys, and you can open a 6 Blue and 2 Black Combo Door by spending 8 Rainbow Keys. Like Master Keys, you activate them for use with some button (perhaps even the same button would switch between using Master, using Rainbow, and neither, since you can only be using one of the two at a time). When you open a door using Rainbow Keys, it acts as if its spend color and all of its lock colors are Rainbow, and of course you must meet all of its requirements in Rainbow to do so, so a door with both positive locks and negative locks (of the same dimension) cannot be opened with Rainbow Keys (...for now). Like with Master Keys, opening a door with Rainbow Keys doesn't change the Glitch color, but since Rainbow Keys do care about the numbers on the locks, they open doors in the same way regular Keys do rather than modifying the copies directly like Master Keys do (for example, a 4 Pink Door with -3 copies, which acts as a -4 Pink Door, will indeed act as a -4 Rainbow Door if opened with Rainbow Keys, and doing so will open -1 copies so it'll have -2 copies left). Doors that have Rainbow or Pure on them are immune to Rainbow Keys. (Notably, Master Doors are not immune to Rainbow Keys...).

Rainbow Keys and Master Doors are different takes on "spending the appropriate number of Master-like keys instead of just one": Master Doors do this by spending actual Master Keys, Rainbow Keys do this by being a Master-like Key that, unlike Master Keys, is affected by lock numbers.
Really, though, Rainbow Keys should get their own world, not be part of this one.

World R5: Holy Attic
A golden room with light shining in through stained glass windows. Despite its holy nature, this church attic has its own curse...

Brown Key variation:
Stained Glass Keys
Stained Glass Keys are a key color that acts as a strange mix of Brown and Glitch Keys. If you have positive real Stained Glass Keys, you have their "blessing": this acts like the Brown curse, except the color it turns doors into is whatever the Glitch color is. For example, if you open a 4 Pink door and then pick up 1 Stained Glass Key, you'll now have a curse that turns doors Pink. Unlike Glitch Keys, the color of the Stained Glass Key curse changes even when those Stained Glass Keys were already in your inventory. Doors with Stained Glass as a color on them are immune to the Stained Glass curse, so Bicolor doors with Stained Glass as the lock color and some other color as the Spend color are often what's used to change the color of the Stained Glass curse. Negative real Stained Glass Keys can undo the curse, but only the curse of that color. This means that if you have negative real Stained Glass Keys and the Glitch color is Pink, they'll undo the Pink curse, but not any of the other curses. Yes, if the Glitch color is Brown and you have negative real Stained Glass Keys, they'll undo the Brown curse even if that curse wasn't from a Stained Glass Key - the Brown Stained Glass curse is the same as the regular Brown curse (this means that Stained Glass doors are immune to the Brown curse too). As with Brown Keys, imaginary Stained Glass Keys don't matter for the curse. Since Pure Doors are immune to the Stained Glass curse, if a door becomes cursed to be Pure, there's no way to undo that curse. The Stained Glass curse cannot be Stained Glass itself, so while the Glitch color is Stained Glass, the Stained Glass curse aura is inactive. Stained Glass doors are not immune to things other than curses, like Master Keys or (if frozen/eroded/painted) Ice/Mud/Ink Keys. If you have both positive real Brown and positive real Stained Glass Keys, you get a "hybrid curse" where the locks of the door become the Glitch color but the Spend color of the door becomes Brown. If you have positive real Brown and negative real Stained Glass Keys and the Glitch color is Brown (so the Stained Glass Keys are trying to undo the Brown curse), the two cancel each other out and you have no curse aura.

Pure Key variation:
Purity Gates
Purity Gates have the Pure color instead of white in their checkerboard pattern. These Gates instill immunities onto the area inside them. Locks of any color on a Purity Gate protect from the curse of that color: for example, if there's a Purity Gate with a 1 Pink Lock and a 2 Orange Lock on it, then the Pink and Orange Stained Glass curse auras don't work while you're touching/inside that Purity Gate. Doors are often placed inside these gates, so you can only open them while the restrictions apply.
If the Purity Gate has Master on it, then you can't use Master Keys while touching it (same goes for other "used" keys, like Checkerboard and Transparent, while touching Purity Gates with their respective color on them). If the Purity Gate has Stained Glass on it, then all curses are disabled while touching it. If the Purity Gate has Red on it then the Maroon aura is disabled while touching it, and same goes for the other five countering aura cases. If the Purity Gate has Glitch on it, then while you're touching the Purity Gate, the Glitch color is not changed even if you open a door that would normally change it. Finally, if the Purity Gate has Pure on it, then all of Pure's immunities apply, so no using Master Keys, no curses, and so on (but the Red/Green/Blue/Maroon/Forest/Navy auras and the Glitch color still work).

World R6: Two-Tone Garden
The tiles of the garden have become a jumble of black and white tiles, with flowers of contrasting color clashing in the background. Perhaps an error was made in the construction of this area of the garden.

Negative numbers variation:
Duality Keys
Duality Keys, similar to Signflip and Rotor Keys, are represented by a symbol where the Key count would be, and in this case that symbol is ±. When you collect a Duality Key, your real count of that Key color becomes "dualized": if you had 6 Green Keys and you collect a Green Duality Key, you now have ±6 Green Keys. In this state, you can open both positive and negative doors of that color, both positive and negative Keys add to your Key count, and both positive and negative doors subtract from your Key count. For example, with ±6 Green Keys, picking up a 1 Green Key puts you at ±7 Green Keys, then picking up a -3 Green Key puts you at ±10 Green Keys, then opening a 5 Green Door puts you at ±5 Green Keys, then opening a -2 Green Door puts you at ±3 Green Keys.
If you had negative of the Key color before it became dualized, then the sign is ∓, which acts the same as ± except for what happens when the dualization ends: when you collect an Un-Duality Key, represented with the symbol +/-, then the dualization ends and that Key color returns to normal behavior. If its sign was ±, the count becomes positive, while if the sign was ∓, the count becomes negative. If a dualized Key color is brought down to 0 (in that real/imaginary dimension), dualization ends early, since 0 doesn't have a sign. If a spending from a Bicolor/Combo Door would put you below 0, then the dualization does not end: instead, ± switches to ∓ or vice versa (for example, if you have ±4 Blue Keys and 6 Blue Keys are spent, you now have ∓2 Blue Keys). Signflip Keys turn ± into ∓ and vice versa.
Dualization for the real part and the imaginary part are separate; imaginary Duality Keys are represented with ±i, and imaginary Un-Duality Keys are represented with +/-i. Rotor Keys can swap where the dualization is: for example, if you have ±6+3i Cyan Keys and you collect a ×i Cyan Rotor Key, you now have -3±6i Cyan Keys, and another ×i Cyan Rotor Key puts you at ∓6-3i Cyan Keys.
EDIT: I forgot to address how some mechanics interact with this, so I’ll do that now. For the purposes of Red/Green/Blue auras and their dark counterparts, a dualized Key amount counts as positive, regardless of whether it’s ± or ∓. For curses (Brown and Stained Glass), on the other hand, the positive and negative parts of their effect cancel each other out, so a dualized Brown or Stained Glass amount does not have any aura effect. “Usable” Keys like Master Keys can be used as either sign, and either way it’ll count as spending 1: for example, if your real Master Key count is dualized, pressing [X] once activates positive Master Key usage, pressing [X] again switches to negative Master Key usage, and pressing [X] a third time puts away the Master Key again - and if you have ±7 Master Keys, using a Master Key of either sign will decrease it to ±6 Master Keys.
EDIT 2: For Bicolored Doors, what number a Dualized Key count counts as depends on the sign of the lock: when you have ±5 Green Keys, a 3 Green lock spends 3, a -2 Green lock spends -2, a positive Green Blast lock spends 5, and a negative Green Blast lock spends -5. For All Doors, they follow the "dualized Key spending goes towards 0" rule and apply it to the Spend Color, so they spend in whatever sign the Spend Color is: for example, if you have ±5 Green Keys and 9 Blue Keys and you open a Green All -> Blue Door, then it spends 5 Blue Keys, but if you have -4 Blue Keys, then it spends -5 Blue Keys.

This post is getting too long now, so the second half is in the replies.

Alright, how about the Backside worlds?

World R7: Starry Seas
Higher up in the sky than before, islands are dotted across the water below and the stars can be seen in the deep purple sky.

Bicolor Doors variation:
Composite Doors
Combo Doors can now have multiple spend colors! A single lock must be in the region of a single spend color: for example, you can have a Combo Door with a 2 Red lock and a 1 Green lock on a White background and a 5 Blue lock on a Black background. This Combo Door will need 2 Red Keys, 1 Green Key, and 5 Blue Keys to open, and opening it spends 3 White and 5 Black.
When it comes to Glitch color, the positions of a Composite Door's spend color matter: the Composite Door's spend colors are "ordered" from left-to-right then top-to-bottom like reading, so the bottom-right-most spending color of the Composite Door becomes the new Glitch color.

I thought of two Star Key variations:
Star-Swap Keys
These Keys have a Star half on the left half of their head and an Unstar half on the right half of their head, and they swap the Star state of that Key color: if it's not starred then it becomes starred, but if it's currently starred then it becomes un-starred.

Starry Keys and Doors
These keys and doors, which can come in any color, have faint star patterns on them (behind the lock(s) in layering in the case of doors). These keys and doors take your Star state and do the opposite: they don't change your count if that Key count isn't starred, but they do change your count if that Key count is starred. Perhaps there are also "Forceful" keys and doors, which have pentagons instead of stars and will always change your count whether it's starred or not, and "Weak" doors which have X's instead of stars/pentagons and never change your count whether it's starred or not (there are no weak keys because they'd do nothing).

World R8: Cliffside Vaults
Pyramids hidden away in the Etchstone Cliffs contain fabulous treasures, but beware of those who guard the tombs!

Negative Master Key variation:
Alarm Keys
These are silver with a red and blue light on top of their head (like the lights of a police car). You can use an Alarm Key on a door to spend what that door would spend without actually opening it: for example, if you use an Alarm Key on a 4 Black Door with 3 copies, your Black count is decreased by 4 but the door still has 3 copies. You don't need to satisfy the door's requirements to use an Alarm Key on it, so this could take your Black count from 3 to -1. Doors with Alarm or Pure on them are immune to Alarm Keys (notably, Master is not immune to Alarm...).
Positive real Alarm Keys spend whatever a positive real copy of that door would spend, even if it doesn't have any positive real copies, so if you have a 4 Purple Door with -1 copies, then it appears as -4 Purple but using a positive real Alarm Key on it spends 4 Purple anyway. Negative real Alarm Keys spend what a negative real copy would spend, positive imaginary Alarm Keys spend what a positive imaginary copy would spend, and negative imaginary Alarm Keys spend what a negative imaginary copy would spend.
Like Master Keys, Alarm Keys do not work on frozen/eroded/painted doors, but they do work on cursed doors (and the curse does affect what the Alarm Keys spend).

World R9: Library Tunnels
A maze of secret passages hidden within the Lockpick Library. Books line the walls of this maze. Which path will you choose to go down?

Combo Doors variation:
Choice Combo Doors
These were already described in my previous ideas post, but I'll paste that description again here.
These are Combo Doors that have a V-symbol on them, like the formal logic symbol for OR. With these Combo Doors, you don't need to meet all of their requirements to destroy them: you only need to meet at least one, and only the locks that you do satisfy will count towards the spending. For example, if you have a Choice Purple Combo Door with a 5 Cyan lock, a 3 Green lock, and a Blast Blue lock, and you have 7 Cyans, 2 Greens, and 1 Blue, then the 5 Cyan and Blast Blue locks will count towards spending since their requirements were fulfilled, but the 3 Green lock will not count towards the spending since you don't have enough keys for it. The Combo Door will still be destroyed since you fulfilled at least one of its requirements, and a total of 6 Purple Keys (from 5 Cyan + 1 Blue) will be spent.
It's possible to nest the two Combo Door types into a single Combo Door: for example, you could have a regular Combo Door where one of the locks is a 3 Red Lock while the other "lock" itself consists of a choice between a 4 Pink Lock and a Blank Black Lock. If a Combo Door has "layers" like this, then those layers will always alternate between regular and choice (because otherwise things that could go on the same layer would be put on different layers), and the outlines that separate the layers will be given alternating colors, where one color is the spend color and the other is a slightly tinted version of the spend color; since the inner layers don't have their own spending colors, only the outermost "locks" can be in different spending colors if the Combo Door is Composite. To make things clearer, if a Combo Door is layered like this, then the layers that are Choice Combo Door will have a V symbol on them and the layers that are regular Combo Door (i.e. where every requirement on that layer must be met) will have an upside-down V symbol on them (the logical symbol for AND).

World R10: Outdated Nexus
An older, outdated model of the Mechanical Nexus. Most of its machines have ceased to whirr, and the lights have mostly gone off. The few machines still active are slowly lagging behind their quotas.

Glitch Keys variation:
Laggy Glitch Keys
This world introduces darker Glitch Keys and Doors known as "laggy" Glitch. The first time the Glitch color is changed, the Laggy Glitch mimic doesn't update, and afterwards it lags behind the regular Glitch color by one: for example, if you open a Pink Door, then a Cyan Door, then the regular Glitch color will be Cyan but the laggy Glitch color will be Pink. If you then open a Purple door, the Glitch color changes to Purple and the laggy Glitch color changes to Cyan. Of course, if you open multiple doors of the same color in a row, then the two Glitch colors will agree, since one door ago and two doors ago are the same color. Laggy Glitch is not a separate color, so if collected on its own, it counts as Glitch, and its mimic will start as Glitch and stay that way after the first door you open, then start changing after the second door you open. Laggy Glitch has the same interactions with Brown as regular Glitch does. Purity Gates with Glitch on them prevent all Glitch color changes: for the purposes of Glitch colors, doors opened while touching a Purity Gate with Glitch on it are skipped in the Glitch color order. Composite Doors trigger multiple Glitch color changes right in a row, so the last spend color of the Composite Door becomes the new Glitch color, the second-to-last spend color of the Composite Door becomes the new Laggy Glitch color. Later in the world, even laggier Glitch Keys are introduced: Double-Lag Glitch Keys (dark green border) are two colors behind, Triple-Lag Glitch Keys (dark red border) are three colors behind, Quadruple-Lag Glitch Keys (dark blue border) are four colors behind, and so on (with different dark-colored borders).

(EDIT, because I forgot these) Lockless Door variation:
Colorless Doors
These doors have locks, but no Spend color. This is probably indicated by the door's exterior being a transparent black, but its locks are not transparent. When a Colorless Door is opened, it doesn't spend any of your Keys. The Glitch color(s) cannot be Colorless, so when a Colorless Door is opened, the Glitch color does not update. (Alternatively, perhaps the Glitch color(s) can be Colorless, but when you pick up a Colorless Glitch Key, no Keys are gained)

World R11: Miracle Chasm
The Miracle Tower overlooks a great chasm at the edge of the world, splitting the world as we know it from the complexities that lie in the Great Beyond. From the balconies of the tower, streams of light can be seen pouring up from the chasm.

Yeah, yeah, I know the quaternions are usually the next step after the complex numbers, but in my opinion, the quaternions would be more trouble than they're worth in I Wanna Lockpick. All three non-real quaternion units square to -1 (so there isn't much gameplay richness added that the complex numbers didn't already have), and the non-commutativity of multiplication would probably make the puzzle logic just too complicated to follow. Instead, I've chosen something more intuitive but also, at least in my opinion, more interesting.

Complex numbers variation:
Split-Complex Numbers
The split-complex numbers are similar to the complex numbers, but instead of the imaginary unit i where i^2 = -1, they have the "hyperbolic unit" j, where j^2 = 1. This world throws these split-complex numbers into the mix, including Split-Swap Keys (×j), Signflip Split-Swap Keys (×-j), Keys and Locks with multiples of j as their costs, and another eyepiece for the Lens of Truth that adds J-View so you can make and view split-complex copies of doors. Split-complex numbers are arguably more intuitive gameplay-wise than regular complex numbers: since j^2 = 1, Split-Swap Keys just swap your real and split-complex counts without throwing on extra negative signs. This means that positives and negatives are more separate than they are with regular complex numbers, which allows for different puzzle designs.
Regular complex numbers still exist, though, so what happens when Rotor and Split-Swap Keys show up in the same puzzle? The system that combines the complex and split-complex numbers is called the tessarines, and it introduces a fourth unit, which I'll call h instead of k to make it clear that these aren't quaternions: i * j = h, j * h = j * (i * j) = i * j^2 = i, i * h = i * (i * j) = i^2 * j  = -j, and h^2 = i^2 * j^2 = -1. Rotor Split-Swap Keys (×h) and Signflip Rotor Split-Swap Keys (×-h) exist, keys and locks can use h just as well as the other three dimensions, and that additional Lens of Truth piece also added H-View. Despite this being a four-dimensional system, multiplication is still commutative.
Yes, h could just be called "ij", but that would be confusing in gameplay because you'd see a door that needs 2ij Black Keys and would probably be confused - naming ij "h" makes it clear that it's a separate unit, even if you often get there by multiplying i and j.
The various things that are separate for real and imaginary (Blast Doors, Master Key types and Lens views, imaginary Checkerboard Keys, Duality Keys, etc.) are also separate for j and h. j Blast Doors are represented via an X with a hole in the middle (so the four diagonal lines are separated), and h Blast Doors are represented via a + with a hole in the middle.

(EDIT, because I forgot these) All Doors variation:
Magnitude Doors
Whereas positive doors have dark locks and negative doors have light locks, magnitude doors have grayish locks. Magnitude doors care about the absolute value of your Key count in that dimension, but not its sign, and they'll spend in whatever sign you do have: for example, a Magnitude 3 Blue lock will spend 3 if you have 3 or more Blue Keys, it'll spend -3 if you have -3 or less Blue Keys, and it won't open if your real Blue Key count's absolute value isn't at least 3. Magnitude doors still only care about their own dimension, so real and imaginary (and split and tessarine) are still separate.
Negative copies of magnitude locks are the same as the original, though imaginary copies are different. An Exact 4 Magnitude lock needs your Key count of that color to be 4 or -4, but is fine with either of those two.
What really makes this an All Door variant is that Blast locks can also be Magnitude, so a Magnitude real Blast Door spends all your real Keys of that color regardless of sign (as long as you don't have exactly 0 of them) but doesn't consider or touch your imaginary Keys - sort of the halfway point between a regular Blast Door and an All Door. A Magnitude All Door would just be an All Door again.

For world R5, I actually saw this idea in the IWL discord that I might try to implement one day. If you get close to a positive cost Peach door, all keys except for Pure and Stone get turned into Peach keys. A negative cost Peach Door turns them back. Converted Peach keys only count for cost and are never spent, so if you have 3 White, 3 Orange, and 0 Peach keys, convert them to Peach keys, spend 3 Peach keys, and turn them back, you'll still have 3 White and Orange keys and you'll have -3 Peach keys.

Okay, but what if we keep going? What about Part 2?

The Vast Depths
The Great Beyond's counterpart. In the entrance area, the ground is more red and pink than the original orange and yellow, the sky is darker, the trees are black and leafless, and red streaks fly up instead of down. You're headed very deep into this world...

Salvage Points variation:
Preservation Points
Preservation Points look similar to Salvage Points, but with an icon of a key inside them instead of arrows. Instead of an ID number, Preservation Points have a color. When you touch a Preservation Point, the next time you enter a level or win a level, whatever your Key count of that color was, that count carries over into the level you just entered/returned to. (The Preservation Point deactivates after being used). For example, if you have 28 Pink Keys, touch a Pink Preservation Point, spend 6 Pink Keys, and enter a level, you'll start that level with 22 Pink Keys, because that's how many you had when you entered it. Likewise, if you have 3 Purple Keys, touch a Purple Preservation Point, and then win the level, you'll have 3 Purple Keys on the world map you returned to. Preservation Points are automatically deactivated if you exit a level, though - if you want to preserve a Key count from a level onto the world map it came from, you have to win that level! Unlike Salvage Points, touching a Preservation Point when you already have one active does not deactivate the first one, so it's possible to carry over multiple Key counts into a single level. Warping with the Warp Rod deactivates all currently active Preservation Points, so you can't preserve Key counts through a warp - you need to rely on the entrances that are actually available.
Yes, Glitch Preservation Points change color like other Glitch objects do, so the color they preserve is dependent on what the most recent Spend Color was when a level change occurs. You definitely can't use Master Keys on Preservation Points, though, and you can't curse a Preservation Point - just because these things have color doesn't mean they act like doors. As for Stone Keys, when they're Preserved, the earned count is added to the Preserved count: for example, if you've currently earned 14 Stone Keys and then preserve 30 Stone Keys, you'll have 44 Stone Keys upon the level transition.

Like The Great Beyond, The Vast Depths is split into chapters, each of which explores Preservation Points in increasing complexity. The Great Beyond's chapters are apparently based on the four seasons, so I'll base the Vast Depths chapters on the four classical elements.
Chapter V1 is Flaming Forest: a forest fire has spread into the Autumn Woods, setting the trees alight in blazing red flame. Sparks and ashes float up into the wind created by the fire. This chapter, as the first Preservation Points chapter, sticks to having Preservation Points within the levels, so you have to win the levels with the right Key counts so you can take those Keys onto the world map and open the doors there.
Chapter V2 is Subterranean Palace: the Frozen Palace has sunken deep underground. Its ice has turned to dirt and stone, and boulders, chunks of ore, and ant tunnels are scattered throughout the walls. This chapter introduces Preservation Points on the world map, so now you have to solve parts of the world map puzzle to bring in the right amount of Keys of the right colors into the levels.
Chapter V3 is Clouded Castle: the Sunshade Castle has risen into the clouds, taking on a more white and blue color scheme, with the ground being made of clouds and wind rushing by in the background. Now entrances to levels start showing up within levels themselves, so you can preserve Key counts from one level into another level, or even into itself!
Chapter V4 is Labyrinth of the Lake: the Dream Labyrinth has been altered to have an "aquatic palace" feel, with waterfalls flowing along the walls, and an underwater background with coral reefs. As with T4, this chapter throws together all of the ways to use Preservation Points from the previous chapters into an advanced, winding, multi-layered (and, given entries within levels themselves, perhaps even recursive) puzzle process.
There's also a Chapter V5: Essential Cosmos, which is based on the fifth element, "quintessence". The background here is black, with twinkling points like stars in the night sky, wisps of various shades of blue, purple, and pink passing through the background, pentagons of various greys (with perhaps some slight color to them) rotating in the background, and a big dodecahedron spinning in the background similar to the rotating cube in Focal Point - the aesthetic of this world is sort of a mix between outer space and Focal Point (with a hint of The Cool Place), but with pentagons instead of squares. Salvages, which have been absent throughout the return worlds, show up here again, and after a few levels that just use Salvages with the new mechanics from the Return Worlds, you reach levels that have both Salvages and Preservation in the mix! The Salvage IDs here are 100 and higher, too high for the Omega Terminal to access, so you're not recoloring these ones.

There's a single color that every Vast Depths chapter has had a Preservation Point of, and only at the end of Essential Cosmos can you get a tessarine amount (i.e. an amount including h) of that Key color. The beginning of the Vast Depths has a Door of that color with h in its cost tucked away, and so you have to get there by retracing back through the Vast Depths, activating the Preservation Point of that color in each chapter in backwards order until you reach the beginning of the Depths to open that door. What's behind that door? World R0, of course!

World 0 doesn't introduce any new mechanics, but I still have an idea for World R0: The Cool Museum, a mix of World 0 and the Lockpick Museum. The ground tiles are the dark blue of the Lockpick Museum background, but the background is a light blue, and the rectangles of World 0 are still passing by in the background (or maybe they're some other shape instead, like pentagons or hexagons). This world's levels each use some complicated contraption to simulate a mechanic that, whether from the Lockpick Museum or from fanmade suggestions, is not in the game, and create a puzzle using that mechanic's emulation.

There are several places things could go from here. If we just want to do Omega Keys again, then there's two ways it could go: the most obvious way is to have the reward for beating R0 be an extension to the Omega Terminal that lets it access the Salvage IDs used in the Vast Depths, and have more Omega Key puzzles to obtain the Checkerboard (and/or Transparent, depending on which one(s) is/are included in R2), Ice, Mud, Ink, Crimson, Forest, Navy, Rainbow, Stained Glass, and Alarm Omega Keys (plus Omega Keys for each level of Laggy Glitch), but it could also go by a different progression: you gain access to all of those Omega Key colors upon beating R0, but now you have to go collect "Numbered Omega Keys", which, instead of unlocking new colors for the Terminal, unlock new Salvage IDs for the Terminal.

However, I have an idea for something different. Omega Keys serve two roles in Lockpick: they're responsible for meta puzzle craziness, and then later on they're used for Chapter EX puzzles. I've thought of a different idea to replace them for each of these two roles, and here's the first one.
Omega Keys, as meta-puzzle goals, variation:
Diamond Keys
Diamond is a Key color with a special trait: changes to Diamond objects are forever. When you collect a Diamond Key, you keep it, even when switching between levels. Likewise, any door with Diamond on it remains destroyed even when switching between levels - if a door changes your Diamond Key count, that Diamond Key count change is also retained, but changes to other Key counts reset as usual (unless Preserved, of course). In other words, your Diamond Key count is always preserved, and Diamond Key pickups and Diamond doors are destroyed permanently... but you can still undo, of course. If you re-enter a level, where all the rest of the actions you did that level are undone but the Diamond actions remain, you can still use Undo to undo the Diamond Key count changes and Diamond door destructions that level caused. For ease of implementation, you can only undo individual actions while they're part of the normal undo chain - once you've left and re-entered a level, if you want to undo its Diamond actions from previous visits, you have to undo all of that level's Diamond actions at once. World R0 would have an entrance to the "Crystal Lab", which has a terminal in it that lets you view all changes to your Diamond Key count, where they came from, and what Diamond doors have been destroyed, and lets you Diamond-undo individual levels from the terminal.
The Diamond terminal would, similar to the Omega Terminal, let you unlock new worlds with enough Diamond Keys - doing so simply requires you have enough, it doesn't actually spend them. The first thing it unlocks, the equivalent to the revealing of hidden Salvage Points, is revealing hidden entrances (such as an entrance from the Chapter V1 map to, say, Page 3 of Chapter V2), letting you jump around between world maps and levels to pull off some crazy Preservation shenanigans, which you'll need in order to get the rest of the Diamond Keys.

So what do those new worlds do?

World R12 / RR1: Return to Return to Doorhaven
It's time to feel nostalgic for feeling nostalgic for World 1

Infinite Keys variation:
Recurrent Keys and Doors
Similarly to Infinite Keys, any Key type can be Recurrent, signified by a looping arrow next to the Key (in the same location as Infinite Keys' infinity symbol). When you collect a Recurrent 1 Red Key, that Key pickup becomes transparent and no longer solid, so you can pass through it freely, but in addition to gaining the 1 Red Key from picking it up initially, every time you open a door (or a copy of a door), the Recurrent Key triggers again, giving you another 1 Red Key. While a Recurrent Key is active, its looping arrow symbol is glowing. Multiple Recurrent Keys can be active at once. Once a Recurrent Key has been activated, it cannot be de-activated.
There can also be Recurrent Doors (which use the same symbol on the corner of the door): a Recurrent Door also goes transparent and non-solid when opened, and every time another door is opened, its spending cost is re-applied (though, with Blast/All/Glitch locks, that cost may be a different number and/or color than it was last time!). If a Recurrent Door's locks are no longer satisfied when it tries to trigger, it fails to apply its spend cost again and it disappears. If a Recurrent Door has multiple copies, it does not become non-solid while it has any copies remaining (I imagine that its exterior remains opaque but its locks become transparent to indicate you can't open them at the moment, and of course the looping arrow is glowing), but it's still activated and performs Recurrent triggers until its locks fail, at which point it reforms and you can open another of its copies. While a Recurrent Door is active, you can't interact with it: you can't open it again, you can't use a Master Key on it, you can't curse it, and so on. If it still had copies left, then it essentially acts like a plain wall until the recurrence ends and it can be opened again.
When order matters (such as when Recurrent Keys of non-regular types, like Signflip or Star Keys, get involved, or when Recurrent Doors are involved... like at all), keep in mind that Recurrent Keys and Doors trigger in the order they were activated. The Glitch color(s) change before Recurrent Keys and Doors trigger, and they do change again after each Recurrent Door opening. Using a Master Key on a Recurrent Door does not activate its recurrence, and using a Master Key does not count as "opening a door" for the purposes of triggering Recurring objects. For the purpose of Salvaging, a Recurrent Door only counts as destroyed when it's fully destroyed, i.e. when a failure to meet its requirements causes its recurrence to stop and makes it disappear. A Key cannot be both Recurrent and Infinite.

Gates variation:
Solidside Gates
What if each side of a Gate could operate separately? This is an idea for expanding Gates' functionality: now, each of the four edges of the Gate can be one of four types: "always closed", "always open", "normal behavior" (closed if you don't meet the requirements, open if you do), or "inverse behavior" (open if you don't meet the requirements, closed if you do). This means, for example, you can have a Gate where only the bottom edge is open if you don't satisfy the Gates' locks, the left and right edges are open if you do satisfy the Gates' locks (but the bottom edge is now closed), and the top edge is always closed. A closed edge of a Gate is solid from the outside, but while you're inside a Gate, the whole thing is non-solid, so the edge rules only restrict what side(s) you can enter the Gate from, not the sides you can exit it from.
To represent this, if a Gate has any non-normal edges, thin lines are drawn on the edges of the Gate: a red line for an always closed edge, a green line for an always open edge, a blue line for an inverse behavior edge, and a black line for a normal behavior edge. These lines are opaque if the edge is closed, semi-transparent if the edge is open. The Gate itself, i.e. the part other than the edge, is always opaque when you don't meet its requirements, semi-transparent if you do meet its requirements. If you're inside the Gate, the Gate and all its edges become semi-transparent.

Chapter VE: Sunset Garden
As your time in the Vast Depths is setting, so too is the sun on the Garden of Dreams. The grass is more yellow-y than before, and instead of a rainbow in the background, there's a sunset with a brilliant gradient of color.

Omega Keys, as Salvage Recoloring, variation:
Epsilon Keys
Epsilon is a key "color" (I imagine them as a dark chartreuse or dark sea green color; perhaps a gradient of greens between the two) with a special property that lets Epsilon Keys behave like Omega Keys, but instead of changing the colors of locks, they change the numbers of locks. Each level in Sunset Garden has an "Epsilon Terminal", which can access some of the doors within that level (perhaps all of them in some levels, but in some levels there are doors and/or individual locks you can't access from the Epsilon Terminal). At an Epsilon Terminal, you can spend 1 Epsilon Key to increase the number on any lock accessible at that Terminal by 1. Likewise, -1 Epsilon Keys can be spent to decrease a lock's number by 1, and same goes for i, -i, j, -j, h, and -h. Once you spend an Epsilon Key at an Epsilon Terminal, you can't retrieve it without undo/restart, but Epsilon Keys are not permanent like Omega Keys are, they and their changes reset after the level like other key colors do. Epsilon is a perfectly valid color for doors and locks.
The Epsilon Terminal will not change whether a lock is Regular or Exact, but it can change the numbers on both types. Locks made to have a count of exactly 0 by the Epsilon Terminal become neither positive nor negative, so they can be opened regardless of your Key count of that color. The Epsilon Terminal still doesn't allow single locks with multiple dimensions of cost, so to turn a 4 Green lock into something involving i, you'll need to get rid of that 4 first. The Epsilon Terminal cannot affect Blank, Blast, or All locks.

The alternative to Bridge to New Memories is "Crystal Culmination", a set of a few levels interconnected with entrances, where Diamond Keys are used to their fullest extent, so despite being multiple levels, it's one big puzzle where some changes carry over across the entire process of solving it. Only the level you start in has a goal in it, but in order to reach it, you need enough Diamond Keys, which you can only get by going through the whole process. Your reward for clearing this puzzle (in addition to, y'know, beating the return worlds as a whole), is the Diamond Omega Key, so once you clear this puzzle you can easily get as many Diamond Keys as you want. (If Omega Keys weren't involved at all in the Vast Depths, then if you didn't get them already, you'd also get the rest of the Omega Keys of the Return World colors here, so you can go mess around with them in the Great Beyond levels)

As for World Omega... World RΩ would probably just be more of the same kinds of puzzles as World Ω itself, but now including the mechanics from the return worlds as well. The name I've thought of for this one is "Further Horizons", and it would be a color-inverted version of the original World Ω, i.e. dark grey with magenta tint.