Well, I personally think that they should have the Miis as fighters. I don't know if they will continue to have Miis by the next release, but I think that they could replace game & watch, or easily become their own entity.
I also think that a character from animal crossing could make its way into the game, with it's final smash being the bees nest.
And this one is more of a pipe dream, but I think that Reshiram and Zekrom could be in the game. But it changes in between forms as one of its moves, so that we could change between the two dragons.
I'm also surprised at no wolf link... seems like it could have had such cool potential