In a video posted by TVTokyo, a Japanese television station that primarily broadcasts anime, posted this video showing Mewtwo's new form.
The Mewtwo's Awakening video teases the upcoming special anime movie dubbed Extremespeed Genesect: Mewtwo's Awakening. The teaser trailer doesn't mention the Pokémon games specifically, but expect some story elements and Mewtwo's new form to dictate new forms for the release of Pokémon X & Y on 3DS.
While you might not understand anything anyone says you can at least get a glimpse of how Mewtwo might change for the upcoming games.