What game engine are you using? I've had some decently crazy ideas about doors and keys, but they'd be better fit for early-game levels, which are already filled. I think it'd be fun to develop ideas by myself.
One potentially useful idea I had was a no-doublejump background block, so that you are forced to, say, fall into the key underneath a door.
I used Game Maker Studio 1.4 with YoYoYo's I Wanna Be the Guy fangame engine as the base for the platforming, saving, and assets. The keys and doors were programmed from scratch, although the visual assets and sounds were pulled from I Wanna KeyPick 100 (decompiled GM8.1 game). I also made a mod of an IWBTG level creation tool to implement the Lockpick mechanics, which was what I used for a large amount of mechanic testing and level design. If you want that, feel free to ask and I'll link it. Directly editing the game itself would likely be very difficult, half because GMS games are hard to decompile (I think UndertaleModTool is the best that currently exists?) and half because GameMaker Studio 1.4 is deprecated software that is no longer available for purchase or usable without a license.
