I think it would be cool to have unlockable or DLC character skins. Maybe make it so that skins are unlocked after your first play through that way you have to beat the game at least once with Lightning. Skins could have 1 or 2 unique powers.
That would be nice if you can use different costumes for the skins too. If all the costumes that Lightning has can also be used on the skins. I would love to play as Fang in a nice sexy bikini.
For skins I would want these the most: 1.Fang 2.Alyssa 3.Vanille 4.Chocolina
I guess her voice would throw it off for the male characters. But it would also be funny. In Saints Row you can use a female voice for male characters and vice versa. It makes it entertaining when you've already beat the game and just messing around on a second playthrough.