Nintendo started accepting other game companies to enter in the game as long as they appeared on a Nintendo syster. Here are some of my ideas for new fighters, from Nintendo of not.
Kalas (Baten Kaitos)
Ray (custom Robo)
Sothe (fire Emblem-Radient Dawn)
Raiden (metal Gear Solid 2)
Banjo (Banjo Kazzoie)
Soma (castlevania-Aria of Sorrow)
Blaziken (Pokemon Ruby/sapphire)