Problem is that it looks too similar to Mewtwo. The facial features, the headtail, they scream alternate forme to me, not a new Pokémon.
I'll like it if it turns out to be Mewthree, but IMO Mewtwo is the KING and deserves a new form, considering other powerhouses have other formes too. Breaking paradigms, I'd say.
Besides, it's just a new forme. If you're one of those genwunners, you can keep your Mewtwo in original forme and use that instead. Depending on what they do with the stat spread, the original might be even more efficient in combat. Therian Thundurus is worse than Incarnate Thundurus.