I was searching for a similar thing but kind of gave up. In particular, I thought this could be a nice tool to give existing characters some variety. I did prototype two scripts which can be used to mix different sprite sheets and recolor patches of character sprites. The problem is, it's not particularly easy to use. To make things worse I wrote it in octave, which nobody seriously uses for something like this 
In the end, I did use it for my own game. But I'm not sure if this thing can be useful for someone else.
Anyway, here it is, maybe it can give you or someone else a bit of inspiration
Recolor/skin character sprites: https://github.com/coscholz1984/Charski
Remix character sprites from multiple input sources: https://github.com/coscholz1984/Charmix
In the end, I did use it for my own game. But I'm not sure if this thing can be useful for someone else.Anyway, here it is, maybe it can give you or someone else a bit of inspiration
Recolor/skin character sprites: https://github.com/coscholz1984/Charski
Remix character sprites from multiple input sources: https://github.com/coscholz1984/Charmix