I'll probably get around to suggesting some new original ideas eventually (or looking into the open-source code and contributing them myself), but for now here's a few things from Lockpick itself that I wish were in the editor:
* Key count displays. Yeah, the PDA exists, but having those wooden billboards does a good job at keeping the relevant key counts in view at all times (and showing which colors are relevant)
* In Lockpick, pressing backspace returns you to the level you came from, at whatever position you were in when you entered the level you're in now. This mechanic ends up becoming required to solve a couple puzzles late in the game (Getting the 1 Stone Door into T4-2, clipping into a door to unlock Focal Point), so it should really be included here - it would make the levelpacks feel a lot more connected. Perhaps there should be an optional toggle to decide which levels you're allowed to do this on, since in Lockpick you can't do this in world maps?
* The camera space feels a little small for larger levels. For bigger levels, shouldn't the level take up the whole screen, like it does in Lockpick? Also, big levels in Lockpick have a camera scrolling option - that'd be useful here.
* Entrances that start out closed and only become open once you've beaten some other level (I'm sure you're already planning this one)
Finally, a suggestion for the editor itself: moving levels around in the levelpack order and duplicating levels. I've already had to manually recreate an entire level because it was in the Level 1 slot but I decided I should probably have a hub in Level 1 so the hub is where you start, and I can see several situations where someone might want to use one of the levels they've already made as the template for another one within the same levelpack, so there should really be ways to switch the order of levels within a levelpack and to duplicate a level within a levelpack. Obviously a proper copy and paste system would be appreciated too (like how Baba is You's level editor has rectangular selection tools), but duplicating whole levels would at least be a good start.