I thought of a couple more:
* Descriptions for individual levels. Right now, the editor supports giving the entire levelpack a description, but I think there should be descriptions for individual levels too, since in Lockpick the levels have flavor dialogue at the bottom.
* Putting doors inside other doors. Lockpick itself only does this in the joke level 0-6, but it does do it, so clearly this is an intended feature. Jokes aside, I think that putting Doors inside Gates, in particular, could be actually useful for puzzle making (due to the "won't go solid while you're inside it, even if you don't meet its requirements" property of Gates), so even if putting solid doors inside other doors isn't added, I think putting Doors (and maybe other Gates) in places that overlap and/or are entirely within Gates should be allowed.