Daisy or Rosalina
Lyndis from Fire Emblem
according to Masahiro Sakurai as the creator and director of the Kirby and the Super Smash Bros. series. and The latest issue of Nintendo official mag is out now with news of new super smash bros wii u and will be featuring sonic and wind waker toon link
I would not be surprised to see the return of Mewtwo in light of his being in X/Y.
Nor would I be surprised to see his Mega forms play a role in his final smash.