We already know Wario's gonna be added, so the only other Mario characters that I wanna see added are Fawful and the Baby Mario Bros.
I can't express any oppinion about Fawful, since I haven't played any game he's in yet. (I know. It's a shame.) However ... the Baby Mario Bros. ?! I know some people really like them and all, but what would their attacks be? Sure, there could be some usage of a Super Star and Baby Mario (Like in Yoshi's Island), but besides that, I can't think of any moves for them.
As usual, I'm dragging in my oppinion on putting Waluigi in the game. This time, I've thought up possibilities for some of the moves in his move set:
Special AttacksB: Pirahna Plant Seed (This one is kind of bizzare. Simply put, he throws down a pirahna plant seed, which grows in moments and creates an obstacle for a short period of time.)
Up and B: Waluigi Tornado (For lack of a better name. I'm talking about something similar to in Mario Power Tennis, when he does his offensive power shot.)
Left/Right and B: Bob-bomb toss
Down and B: ???? (Still haven't thought of one for this)
Regular AttacksA: Rapid Kick (Repeatedly press A)
Up and A: High Kick
Left/Right and A: Double-fist punch (Uses both arms, one after the other, to punch in the indicated direction)
Down and A: Sliding kick
And for the sake of entertainment ...
Taunt: A certain 'bold victory dance', seen only in Super Mario Strikers. Bwahahaaa. (Note: This is a joke ... sorta ...)
So there you have it. Some ideas on why he could make a killer character. To balance out my post, however, I gotta say that if I'd vote for any characters NOT to make a second appearance, it would be Pichu, Mewtwo, and Ice Climbers. I feel like there were more, but I honestly can't remember them at the moment.