First off, you're mixing canons, stop it.
Secondly, it's not a new forme, but rather an entirely new Pokemon.
Third, I hope it's something like this:
>Team Plasma somehow acquires some of Mewtwo's DNA
>The DNA doesn't match up to any Pokemon that their scientists know of, because Mewtwo is essentially non-existent to the rest of the world
>Some comment about the genes in the DNA sample being "Berserk" is made
>Since Plasma's attempts at reviving and modifying Genesect were so successful, they decide to try cloning a Pokemon from the DNA
>Since the DNA they poses is that of Mewtwo and now directly from Mew(inb4 they're the same thing)the end result of the cloning process is Newtwo
>While in its tank, Newtwo forms some sort of Psychic link with Mewtwo, like Mewtwo did with Ambertwo and the Starterstwo
>Mewtwo flies off to Unova to see what the deal is, and to prevent Newtwo from making the same mistakes it did
As for anime, I hope it's the same thing, but while Mewtwo is flying around Unova trying to find Newtwo, he spots Ash and co. about to get their shit smacked so he intervenes, then Mewtwo and Newtwo fight the horde of Genesect together.
Don't forget, guys, there's still another Mew-related clone or forme or whatever to come.
