I was looking at relrel's project, and I saw he had custom dropdowns for variables and lists.
I played around with the .json and found you can use %m to make dropdowns for sounds, broadcasts, and costumes, too! He also told me that there are some options for sprites.

Here's what I have so far:
%m.list
%m.var
%m.broadcast
%m.costume
%m.sound
%m.spriteOrStage
%m.spriteOnly
%m.spriteOrMouse

This should be implemented as a feature, IMO.
So are there any others? Post below!
There are probably options for all dropdowns, like rotation style, color effects, backdrops, operators, key names, etc!

There's already a topic about this here, and I made a list of all the input specifiers here. I'm going to close this because there's already a fair amount of discussion on that other thread.