I completed the true final puzzle a few days ago, and now I have a few ideas for additional gimmicks that I think could make for good additions to this game. Yes, I'm aware that the development of the game is far enough along that it's too late to make such suggestions and actually have a chance to see them in the game, but I might as well post these for fun anyways. Unless otherwise specified, these are door properties, not door colors, meaning they can be applied to doors of any color and do not have a color or associated key of their own.
Exact Doors
Doors that only open if you have exactly the right number of keys: an 4 Orange Exact Door can be opened if you have 4 Orange Keys, but not if you have 5 Orange Keys. When it comes to Combo Doors, the Exact property is applied to individual locks rather than the door as a whole.
Linked Doors
If two doors are linked (which will be indicated by them both having a glowing colored circle on them), then when you destroy one door, the other door is destroyed too and also applies its cost, even if you don't meet its requirements. Whereas Combo Doors allow for doors that have requirements in multiple colors but can still only spend one color, Linked Doors effectively allow for a door that spend multiple colors, such as having a 4 Purple Door linked to a 3 Orange Door, so if you destroy one of them, they're both destroyed and 4 Purples and 3 Oranges are spent.
Linked Doors aren't limited to a single set of two, of course: there can be multiple sets of Linked Doors (destroying a Linked Door only destroys the other door(s) in that same link set: different Linked Door sets are indicated with different colors of link circles. The link circle colors have nothing to do with the actual colors of the doors, they're just different colors to indicate different link sets), and a link set can contain more than two doors (for example, if three doors are linked to each other, then destroying any of the three doors will destroy all of the three doors).
Using a Master Key on a Linked Door will still destroy every door in that link set (and the same idea applies to the other types of Master Key, so using a negative Master Key on a Linked Door will add a copy to all doors in that link set). Aura interactions, on the other hand, apply to the individual doors in the link, so unfreezing a door will only unfreeze that door even if it's linked to other frozen doors. Frozen, eroded, or painted doors will still be destroyed if a door they're linked to is destroyed... as long as the door only has one of those effects. If a door has two or all three of those effects, it will not be destroyed even if a door in its link set is destroyed. Cursing a door will remove any link circles it has, and those links will still be gone even if the door is un-cursed.
For example, one way to use Linked Doors in a puzzle would be to put some of the doors in the walls (like that Purple Door in 8-1), except now they actually do something since they're effectively adding an additional spending effect to whatever door within the accessible puzzle that they're linked to. Another way to use Linked Doors would be to have a puzzle where there's a chain of doors blocking the checkmark, and you have to figure out which of those doors to destroy directly and which ones to destroy via the door(s) they're each linked to. There are probably quite a few ways to use each of my suggestions, but I'll try to stick to one or two examples of each here. If you have any ideas on how any of my properties could be used in a puzzle, feel free to put them in the comments.
Portal Doors
These also come in sets of two or more. When you destroy the first Portal Door in a pair, it drops a (semi-transparent) portal at the location it was in. When you destroy the second Portal Door in a pair, then upon destroying it, you're teleported to that portal from the first Portal Door in the pair.
Portal Doors don't have an innate order to them, so you can destroy them in either order, and whichever one you destroy second will teleport you to the portal from whichever one you destroyed first. Standing in front of the portal doesn't do anything, so the only way to use it is to destroy another Portal Door in that portal set. The second door in a portal set you destroy does not drop a portal, only the first one in that set, so if there are more than two doors in a portal set, than each of the doors in the set will teleport you back to the portal from the first one you destroyed in that set. If there are multiple sets of portal doors, the first Portal Door you destroy in each set will drop its own portal, and destroying a Portal Door will teleport you to that set's portal (it won't teleport you if that set hasn't dropped its portal yet).
Using a Master Key on the first Portal Door in a set can still make it drop its portal, but using a Master Key on one of the other doors in that portal set once there's a portal will not teleport you to the portal, since (as established in World 10) it doesn't really count as you destroying the door. Negative Master Keys only add copies to one of the Portal Doors rather than both (unless the Portal Doors are also Linked), and the first door won't drop its portal until it's completely destroyed, but once there is a portal for that set, you'll be teleported to it each time you destroy one of the other doors in that portal set (even if you only destroyed one of its copies): dropping a portal requires a door to be reduced to 0 copies and disappear, while teleporting to a portal simply requires you to destroy another door in the portal set (even if it's not completely destroyed).
If two Portal Doors are also Linked, then if you destroy one (thus destroying both because they're linked), the other one drops the portal and you're teleported there. There are certain situations with Portal and Linked Doors where a portal might not be dropped: if you destroy multiple Portal Doors in the same portal set (such as via a Linked Door) in one go, you'll still be teleported to the portal if there is one, but if there isn't a portal, then unless the door you destroyed in-person is one of the Portal Doors and there's exactly one other Portal Door in that set that you destroyed in that action (see the previous sentence for that case), no portal will be dropped, such as if you destroy two Portal Doors of a set in a single action but neither of them are the door you destroyed in-person, or if you destroyed three Portal Doors of a set in a single action (even if one of them is the one you destroyed in-person) and there isn't already a portal for that set; if such an interaction causes a portal to fail to spawn, but there are still Portal Doors in that set left, one of them can still be the one to drop the portal. If a Linked Door interaction would try to teleport you to multiple portals at once, then none of them teleport you that time (but a Linked Door interaction can drop multiple portals from different sets).
For example, Portal Doors could be used to make more puzzles that use your jump height as a mechanic (like the Rickety Bridge puzzles and 10-B do), as well as puzzles that are split into sections that you can't return to once you leave (Linked Portal Doors would be a good way to make such a one-way passage).
Temporal Doors
These come in sets of two, and unlike Linked and Portal Doors, Temporal Doors cannot come in sets of three or more, just two (though there can still be multiple sets). When you destroy the first door in a Temporal pair, nothing special happens, you just destroy the door as normal. However, if the one Temporal Door is already gone, then when you destroy the other one (even if it's just destroying one of its copies, but of course using a Master Key does not trigger this), the state of the level (your location, doors opened, keys picked up, etc.) revert back to right before you destroyed (the last copy of) the first door in the pair, as if you used undos to go back to that point... except the two Temporal Doors themselves remain destroyed, and your key inventory is not undone, so if you had, say, 4 Purple Keys when you destroyed the first Temporal Door but had gotten up to 7 Purple Keys by the time you destroyed the second, you'll still have 7 Purple Keys after the reversion.
There's also a catch to the Temporal Doors being destroyed: if the remaining one still had copies left after you destroyed one of its copies, then it will retain those copies, and its temporal partner will be brought back (with just one copy). If there are multiple temporal pairs, then triggering a reversion will only leave the pair that triggered it destroyed: any other doors that are brought back by the reversion will still be brought back even if they're in other temporal pairs.
If the doors of a temporal pair are also Linked, then destroying the first one will also destroy the second, which causes an immediate reversion (but since those two doors remain destroyed, this reversion only has an effect if there are other doors in that link set, because they'll be immediately brought back by the reversion). If multiple temporal reversions are triggered at once (destroying multiple temporal doors in a Linked Door destruction), then the reversion that goes back the farthest is the one that goes through, but all of the temporal pairs that tried to revert from that one action remain destroyed. If the second temporal door is part of a portal set that already has a portal, then you'll be teleported to the portal after the reversion (if the portal still exists once time has reverted). If two doors are a temporal pair, part of the same portal set, part of the same link set, and there is no portal for that portal set yet, then destroying one will immediately destroy the other, causing a reversion but also opening a portal at the second one (because that's how Linked and Portal interact), so you'll be teleported to the second one right after the reversion.
For example, Temporal Doors could be used to make recursion-based puzzles, where you have to go through the same doors multiple times (via forcing you to use Negative Master Keys on one of the Temporal Doors) until you meet some sort of escape condition (such as running out of, say, Pink Keys so you can open a Blank Pink Door to get past the part of the puzzle that's forcing you into the loop).
Ghost Doors and Phantom Doors
Ghost Doors are my name for a suggestion that I've seen before: doors that you can walk right through without destroying them if you don't meet their requirement, but they will be destroyed if you do meet their requirement. For example, if you only have 3 Pink Keys when you walk up to a 4 Pink Door that's a Ghost Door, you can walk right through the door without destroying it, but it will be destroyed if you do have 4 Pink Keys when you walk up to it.
Phantom Doors are sort of the opposite of Ghost Doors: they're solid so you can't walk through them, but when you do destroy a Phantom Door, it doesn't actually spend the keys used to open it. For example, a 4 Orange Door that's a Phantom Door won't let you through if you don't have at least 4 Orange Keys to destroy it, but when you do destroy it, you don't actually spend any Orange Keys.
The names Ghost Door and Phantom Door are similar on purpose, but if their names are too similar, perhaps Phantom Doors could be called Illusion Doors instead?
Ghost is a property that's applied to a whole door, but Phantom can be applied to individual locks of a Combo Door, allowing for some locks (the non-phantom ones) of the Combo Door to actually affect the spending color, while other locks (the phantom ones) are simply requirements for opening the door that don't count towards the spending. A door can in theory be both Ghost and Phantom, but that basically makes the door do nothing at all since it doesn't block or spend anything... although even such a useless door can have a use once my other suggestions like Linked Doors, portals, etc. get involved.
For example, Ghost Doors could be used as a sort of "background element", as if they're a smog occupying part of the level that you can walk through, but if you meet their requirement then they'll be destroyed and spend your keys. Phantom Doors would probably best be used as simple gates that let you through once you meet a requirement, or as requirements to open a Combo Door that don't count towards its spending cost.
Choice Combo Doors
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 Combo Door with Purple as its spending color, 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, or you could have a Choice Combo Door where one of its locks is itself a regular combo (i.e. where every requirement in the combo must be met) and one of the locks in that layer is itself a choice combo (where only one of the requirements there must be met to satisfy that lower choice combo), and so on. 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; after all, a Combo Door still only has one spend color. These "layers" only let you nest choice Combo Doors and regular Combo Doors (like nesting ORs and ANDs); you'll still need to use Linked Doors to effectively have multiple spending colors. To make things clearer, if a Combo Door is layered like this, then 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).
Negation
A Negated Door has a glowing red border, and its conditions for destruction are inverted: the door will only be destroyed if you don't meet its requirements, and its spending cost is inverted. For example, you can destroy a Negated 5 Black Door if you have less than 5 Black Keys, and doing so will give you 5 Black Keys instead of spending 5 Black Keys.
A Negated Blank Door acts like an All Door, and vice versa.
A Negated Blast Door can only be destroyed if you don't have any Keys of its respective type (so a positive Orange Blast Door can only be opened if you don't have any positive Orange Keys; it's fine if you have negative and/or imaginary (of either sign) Orange Keys), and instead of spending all your keys of that color, it gives you all your keys of that color, meaning it effectively doubles your keys of that color (so if you open a Negated Orange Blast Door while you have -2 Orange Keys, you'll now have -4 Orange Keys; if it was a Combo Door that contained two Negated Orange Blast locks, then you'll go from -2 Orange Keys to -6 Orange Keys, since each lock gives you -2 Orange Keys. Of course, real Negated Blast Doors still don't affect or care about imaginary numbers of keys, and vice-versa).
Combo Doors can have individual locks negated and/or the door as a whole negated (and/or particular layers for a layered Combo Door): in the case of the latter, keep in mind that if the whole Combo Door is negated, you can destroy it as long as you don't meet at least one of its requirements, whereas if a Choice Combo Door is negated, you can only destroy it if you don't meet every one of its requirements. Negating every one of the individual locks of a Combo Door therefore has a different effect than negating the door as a whole.
A Negated Lockless Door can't be opened at all.
There are also Negation Keys, which you can use with a button like you can use Master Keys (perhaps with N) to negate a door. If you use a Negation Key on a door that's already negated, it returns to being not negated. While Combo Doors can have individual locks or layers negated as part of the puzzle, Negation Keys are only capable of negating or un-negating the door as a whole. Negative and Imaginary Negation Keys do the same thing as positive Negation Keys. Pure Doors, of course, are immune to Negation Keys.
If Negation Doors (doors that have Negation Keys as their required color) exist too, then they have a few special properties. Similarly to how Master and Pure locks work, a door with Negation as any of its locks or its spending color is immune to Negation Keys. When you destroy a door with Negation as its Required Color, then any locks in the level that have the same Required Color as the Spend Color of that Negation door will be negated: for example, if you destroy a door with Negation as its required color and Orange as its spend color, then any Orange locks in the level (whether they're their own doors or individual parts of a Combo Door) will be negated (or un-negated if they're already negated). This only happens if the amount of Negation locks on the door (in the Combo Door sense, so a door with a single 2 Negation lock counts as having one Negation lock, while a door with two 1 Negation locks counts as having two Negation locks) is odd: if it's even, then the negations cancel out and this effect does not occur. Pure and Negation locks are also immune to this effect (which means this effect does nothing when you destroy a regular Negation Door, it has to be bicolored/combo for the effect to do anything), as are any locks on a door with Pure or Negation as its spend color, but since this effect affects individual locks (whereas Negation Keys affect the door as a whole), a Pure or Negation lock on a Combo Door is not enough to protect other locks on that door from this effect.
With regular Combo Doors (AND), Choice Combo Doors (OR), and negated locks and layers (NOT), there can be puzzles that contain very complicated Combo Doors with several layers of logical statement.
I have at least one more idea for another gimmick, but it's not a door property, so I think I'll save that one for another post. Were these good ideas? Which one was your favorite? Do you think these would fit in with the rest of I Wanna Lockpick's gimmicks?