At the very end of the Nintendo Direct they showed off a new
trailer that focused on Charizard coming back as the 28th playable
character. While the trailer showed him off being 100% awesome, they also showed a new opponent hiding in the shadows. It was revealed that a new fighter has entered the ring—Greninja, one of the new Pokémon from
and Y will be a playable fighter in the next game as well. Charizard
will also be able to transform into his Mega Evolution, which will
probably be a Final Smash of sorts.
Source: Nintendo Direct