I'd like to see as few cuts as possible, even possibly the return of Dr Mario and Young Link through alternate costumes on Mario and one of the Links. Wario style anyone? If Mewtwo returned with a complete awesome redesign I'd be a happy camper. Also ROY.
I'd like to see the Pokemon Trainer get revamped, possibly replaced by a cross-generation trainer who uses a Gen I, a Gen III, and a Gen IV or Gen V Pokemon to round out the representation. mean, we already have Pikachu and Jigglypuff for sure, and Hopefully Mewtwo as well. Other gens need more love. My fanboy self wants to see Blaziken make the roster at least in this way, but I could see Zoroark in there too. How they incorporate Illusion is anyone's guess, though...
More than anything, however, I freaking want Bowser Jr in SSB4 ._.