I have sort of a late-breaking news bulletin to report. The latest trailer for the next Pokemon movie in Japan has been released, and it does confirm that Mewtwo does indeed have a new form. Take a look.
According to serebii.net, the new form is being labeled as Awakened Mewtwo. But it has yet to be confirmed as the official name of the form. I'll post more info when I learn more, so stay tuned.
UPDATE: Nothing new on Mewtwo's Awakened form, but the details about the film's plot have been revealed. Apparently the five Genesect featured in the film have escaped Team Plasma control and are ticked off to find their home from the prehistoric age destroyed and overrun by humans and other Pokemon, thus deciding to reclaim it for themselves. Mewtwo senses the actions of the Genesect quintet and sympathizes with them, as it too was a Pokemon created by human hands. Despite this however, it seems that Mewtwo is going to stop the Genesect from reclaiming their homeland.