I would love to see the following feature added to the sprite editor:

Keyboard shortcuts for changing the active drawing tool. This would be especially useful when editing in 'full frame' mode and the tool buttons are not visible.

Suggestion:

p - pen
s - stamp (Note: currently used for select, see below)
f - fill (Note: currently used for flip, see below)
c - circle/oval
l - line
b - box (also know as rectangle, but r is already used for rotate)

Notes:

The s key is already used for select in pen mode, but I never use it, because it's the same as the Shift key. The key could be re-assigned IMHO.

The f key is already used for flipping horizontally. But it's the same as the H key. The key could be re-assigned IMHO.

More ideas:

Because in full-view mode, you can't see the tools, it would be nice to show the active tool in the bottom status bar (or perhaps in the top bar?). As a bonus, why not show the currently active color there too?