Zack Fair (since the two appear in the same game FFVII, they belong to the same company and for whom it would be an indirect advertising towards the FFVII Crisis Core)
I wouldnt care about the voice.. much. i mean it'd be nice if they did but it's no biggie imo.
alot of games that have multiple characters as skins leave the original voices when you are in a different skin, like recently prototype 2 has alex mercer as a skin yet it is still james hellers voice
What about alt costumes for characters like lightning or sephiroth?
worse, becouse they have a very diffrent fighting style. Eihter way, everyone who love this characters would feel only hate. xD