Hi all,
I've decided to tinker in game development by making an RPG from the ground up and I'm currently in the brainstorming requirements phase and was wondering if their is anything obvious systems or elements that I may have missed. Kinda just probing for insight and suggestions for user development stories that a beginner like me might have missed.
1. Character Movement
2. Scene Transitions
3. Interactable NPC/ITEMS
4. Battle System
5. Inventory
6. Dialogue System
7. Procedural Dungeon System à la Dark Cloud 2
8. Over world monsters
9. Day/Night Cycle
10. Weather systems
11. World map/Fast Travel
12. Party/Classes
13. Saving
14. Leveling System
15 Puzzles

RPGs are pretty involved, so for your first one I recommend keeping it to the bare essentials.  You can always add more in your next game.  I bit off way too much in my first attempt at an RPG and ended up giving up on it.  What sort of RPG do you want to make exactly?  The must-have features will vary from one subgenre to the next.  The post-mortem for my first released RPG talks a lot about scope, so you might find it helpful.  Here are some thoughts on your list, assuming it's for a fairly traditional RPG:

3. Interactable NPC/ITEMS

NPCs are nice to have, but not strictly required.  The more detailed the interaction, the more work it will be.  Things like shops or dialogue trees are much more involved than people-shaped objects that print static text.

4. Battle System

Turn-based, ATB, or action combat?  If turn-based, what kind of initiative rules do you want (e.g. up-front input, round-robin, phase-based, interleaved...)?  Decide what kind of combat you want, then decide what kind of stats you will have to support it.  Then start working on the math for how those stats actually work.  For example, how does Strength affect your standard attack damage?  How do weapons and armor affect it?  What's the expected difference in melee damage between a warrior with a greatsword and a wizard with a knife?  Creating some spreadsheets can help here, but the simpler you keep it, the less time it will take to figure out.  Writing your rules for mental math like you would with a tabletop game can make them easy to build and easy for the player to understand, but simpler formulas may be more likely to break down when applied to a wide range of scenarios.  Try writing down what you want your results to look like in different scenarios, then work backwards from that to figure out formulas that produce those results.

6. Dialogue System

Definitely optional.

7. Procedural Dungeon System à la Dark Cloud 2

Unless you are specifically making a roguelike, this is completely optional and a lot more work than you might think.  The difficulty I've faced with procedural level generation is that it needs a lot of "stuff" in its toolbox in order to make even slightly interesting layouts, and that stuff takes a lot of time and effort to implement.  I'd say stick with hand-designed dungeons for now.

8. Over world monsters
9. Day/Night Cycle
10. Weather systems
11. World map/Fast Travel

All of these are optional.  Even the world map isn't strictly needed; you can have an RPG set in a single location / dungeon (which can also keep your scope down), or you can handle travel to different locations with a simple menu.

12. Party/Classes

I personally find party-based RPGs a lot more interesting than solo character ones, but your mileage may vary.  At any rate, this is something that can get as complicated as you want it to be.  In MM0, I had five predefined characters with only loose and hand-waved class mechanics.  I didn't have time for anything more than that, but my next game will be a lot more robust.  The question that is probably most important to answer early here is: how much agency does the player have in shaping their character(s)?  Remember, the more options the player has, the more time you will have to spend playtesting those options.

14. Leveling System

This ties into what I mentioned before about stats and formulas.  Since this is a key part of your game, you'll want to have a good idea of how you want this to work ahead of time.  Some things to think about: linear XP growth or exponential?  Linear power growth, or exponential?  Is grinding levels possible, and if so, how much do you expect the player to do?  Is leveling player-guided (which means more work for you, both in implementing and balancing it) or automatic?  How important are levels and stats vs. equipment?

15 Puzzles

These are fun, but depending on what kind of puzzles you want to add, this could be an entire set of mechanics by itself.  For example, puzzles based on tool use and environmental manipulation (Lufia II, Zelda) would be a lot more complicated to implement than simple logic puzzles or riddles.

One thing you didn't mention is GUI / menus.  For RPGs, this can get to be a lot of work.  Every system needs a way to interact with it, so keep that effort in mind when scoping out possible features.  Also think about whether you want the game to be playable with a gamepad, or just mouse and keyboard.  Gamepad support is a great accessibility feature, but you need to keep in mind how they work when you design your interface.