April’s CoroCoro has leaked, confirming that the “new Pokemon” is indeed a new forme of Mewtwo. On the third page below, it compares the old and new versions of the 16th movie’s posters, saying “Mewtwo, who was in the background, has awakened and become a new forme!” Thanks goes to Gin for the translation! The pages do not contain any other new information.
The issue also includes the first part of the 16th movie’s manga adaptation, some of which you can browse through below. Since we see a flashback to Mewtwo inside its test tube, we can probably assume the movie will recount at least some of the first movie. We will find out more about Extremespeed Genesect: Mewtwo’s Awakening on April 18th when a new trailer for the film is set to air.