Ms.Fortune has a palette that references a anime character I like a lot : Black Rock Shooter, basically a black-haired, pale-skinned girl.. with a huge cannon. And to boot, if what I read was correct they will share the same voice actress in the Japanese arcade version of Skullgirls.
I guess BRS wouldn't look too out of place in Skullgirls, and her cannon would probably making her very fun to use. Explosions everywhere!
Buuuut she's not a video game character, so I guess she wouldn't fit as a guest fighter.
Maybe Chun-Li? She's one of the most well known girls from Street Fighter, one of the most famous fighting games.
And if all else fails, I'd honestly be okay with having the April Fools' joke, Zone-tan, as an actual fighter. I'm not kidding.