A good example of this might be the UFC games - they let you create a fighter, add tattoo's, body shape parameters etc etc - then you upgrade your character. Then, once the single player game is complete with that character - he retires and is only available in some of the game types. This means that you can fight among upgraded characters seperately, and is actually one of the more interesting aspects.
I guess that making changes to the skin is easiest, adding tattoo's etc etc, but making changes to actual geometry might be a bit of a stretch. I mean, I really think that if it can't be implimented easily, it'll eat up a lot of your development time for a fairly measly reward. I'm not sure what options you have in Unity for making changes to characters. In DBPro, I would probably use vertex adjustments, or scale bones, and it would still be damn difficult I think. I would suggest doing some experiments first, see how you can randomly affect a characters proportions, if animation still works after that, if texturing is still good, that sort of thing. After that you'll know yourself just how much you can sensibly change.
Maybe it would be enough to just customize a character, not the actual mesh but the textures, add tattoo's, change colours, add patches to clothing, change hair maybe. That would be a lot more straightforward and would have no chance of detrimentally affecting your animations or models.

I got a fever, and the only prescription, is more memes.
