I just recently got Pico-8 and I'm already a huge fan. I have a few suggestion of things that could hopefully rather easily be implemented into the editors which might help some people, such as myself :)
The ability to scroll the map with the arrowkeys. I find it a bit cumbersome to use space and drag around the map, especially since the map window is so small if the sprites are visible at the same time.
Be able to switch "sprites" by right-clicking on them. This might be a little more tricky but it's a feauture I really like from Charpad (C64 map and tile-editor). If you right click on a sprite it will swap place with the currently selected sprite and "remap" them on the map. This is just for tidyness - makes it easier to move things around.
Make "delete"in the music editor work aswell as backspace, and make one of them step down one step like the editor does when placing notes.
Make delete in the sprite editor clear the currently selected sprite to the currently selected color.
- Adding keyboard-shortcuts for the draw and fill-tools in the editor.