I don't know if anyone's said this, but I'd give my left but to see Geno (SMRPG) in the game. And he'd have the move where he throws a series of discs that can deal max damage if you press the button again at just the right moment.
Sticking to SMRPG, if Peach is included again, her "limit break" move should be "psycho bomb": she tosses a single bomb into the air and 20+ bombs rain back down on the enemies.
As far as "clones" go, characters like Fox and Falco should just exist as alternate costumes. And as long as Star Fox is the subject, another character they could add would be Wolf (alternate costume being Panther). Just what I would like to see.