I didn't think that poster leak was real until now, or if I'd even like it. But after seeing the footage, I've grown attached.
Still unsure if it's Mewthree or Mewtwo's Awakening Form though. It does seem to be treated as a brand new Pokemon rather than a new form, so Mewthree is the most obvious of the two. Though they could be covering that detail up that it's a new form, as it does seem to be appearing in the new movie, according to that poster. Both are really good possibilities.
IMO, this also pretty much confirms the genetics theme of X/Y. The evil team of the game could've experimented on Mewtwo, giving him this form, or cloned Mewthree from him.