Reviving this thread because we got a lot of new info.
We got details on Diancie's Mega Evolution! It has the ability Magic Bounce and a stat spread of 50/160/110/160/110/110. It also gets Earth Power and Heal Bell from ORAS tutors!
Hoopa also has an alternate forme triggered by an unknown Key Item. It's typing changes to Psychic/Dark and its stats get boosted to 80/160/60/170/130/80. Because its an alt forme, it can hold items, and it gets a new signature move:
Hyperspace Fury - Dark Special - 100 BP, never misses, 5 PP.
Hoopa also gets tons of new Tutor moves like Knock Off/Gunk Shot/Elemental Punches/etc.
Discuss! Bear in mind that Hoopa is still not released yet, unlike Diancie.