Pokémon X and Pokémon Y

The description here makes it highly likely to be a new Pokemon :S

I was thinking that too.

Plus the official video even says "A new pokemon with a familiar look" AND "Get a glimpse of a newly discovered Pokémon in Pokémon X and Pokémon Y that bears an intriguing resemblance to the Legendary Mewtwo!"

And it was announced that the reveal was going to be a new pokemon, not a new forme.

To me it seems almost certain this is a brand new pokemon. So I don't think people should be alarmed at them changing a classic pokemon.
I've wanted a MewThree since gen 1, pretty much only because the genetics of Mew and Mewtwo made further improvements possible. The existence of a Mewthree could be easily explained through Genetics. Even though Mewtwo was powerful it wasn't the perfect being they expected, so it makes sense they would make a more "perfect" version, hence why it looks more like Mew.