What I feel is the major thing holding back the image editor is the lack of color picker, as well as the crop.
I suppose per-pixel collisions wouldn't hurt, either (though maybe that already is there and I can't tell since every sprite NEEDS to be a big box).
I mainly say this because with color picker, you can remove the filler color around the sprite you make (say if you made it in paint), and then with the crop you can have the sprite be as big as it needs to be- not as big as the original box it came with.
I suppose this is queued somewhere down the line, but with color picker, crop, and per-pixel collision that comes with it, I think C2 would be well on its way towards highly feasible functionality.
Unless, of course, there is a better way to make sprites that I am unaware of, that solves the large-box issue.