Before we learned that Lucario also had a mega form I would've said Mewtwo had a pretty good shot. Now I don't know. They could add both but I kinda doubt it.
It's gonna be interesting to see what they do with the Pokemon roster. I'm expecting at least
Pikachu
Jigglypuff
Mewtwo (with Mega final smash)
Lucario (^)
Pokemon Trainer (Squirtle, Ivysaur, Charizard)
man there is way too much gen 1 representation. I wish they would use Pkmn Trainer as a way around it but it's just more gen 1 Mons. I know it's the most famous and iconic and all, but it's so incredibly lopsided it isn't even funny.
Ah well.