What I think it should be is a Pokemon trainer that can be customizable with their customizable pokemon costumes. (i.e. blue Fennekin with Johto trainer). I think this is completely possible because if I can remember, Masahiro Sakurai or someone announced that the new Super Smash Brothers will focus on more detail and moveset, and less 'more characters'. So basically, they are choosing quality over quantity with the new one, therefore costumes with more originality on being more customizable is ideal. I think they should include all trainers (maybe girl ones too if they have the time, but don't count on it) Kanto, Johto, Hoen, Sinnoh, and Unova. This would make a lot of people happy. Also, keep Pikachu and if you're keeping Jigglypuff, don't screw it up like you did in Brawl.
Sorry for the novella.