Maybe it'll be Mewtres? (Mew+ Spanish for three) no that sounds like Moltres...maybe Mewtrois? (Mew+french for three). I know still number, but if they do another language it'll seem more original than making it three...if it is a new Pokémon. Now if it's forme I'm guessing they'll be called Mewtwo X and Y, and maybe be like Deoxys during Gen IV, having their forms depend on the version, but with the ability to get the other by using keys or something that has to do with linking with the other version.