A giant enemy crab.
In seriousness, I'll be happy as long as Yoshi, Ness, and Pichu return. I hope they don't make too many changes to Ness... I may "pick up" a new character or two, Wario, perhaps.
There are sure to be at least a couple playable Hoenn Pokémon. I personally think Combusken is cooler-looking than Blaziken and would prefer it, because Combusken looks like a deadly fighting chicken (which amuses me) while Blaziken looks like nothing I've ever seen before.
Don't care whether Sonic is in it or not. If he is, he's there, whatever. I haven't played any Sonic games and couldn't use him anyway (Something, if I haven't even played their game I feel like it's not my place to play as the character.). If he isn't, oh well, whatever.