CoroCoro scans have revealed that the new Mewtwo forme and Mewtwo are indeed one in the same. New movie posters say in the background say that Mewtwo has re-awakened with a new forme!
No other new information was revealed in the leaked CoroCoro

Theory:
Based on the trailer (How Mewtwo was able to intercept Genesect using Extremespeed), added to the fact that all the characters as well as Genesect where slowed down significantly while Mewtwo was moving at average speed. It's possible this new forme is a sort of "Unleashed" forme, that gives Mewtwo greater power (Example being possibly time manipulation). It could be GameFreak's idea of allowing Mewtwo to regain his glory as the most powerful Pokemon (since he shares the exact same stat total as Ho-Oh, Lugia, Giratina (both), Dialga, Palkia and Rayquaza, losing to Arceus by 40 points) in existence.

Just a theory/

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