I think I have an alternative hypothesis for the Greninja. Ash's Frogadier's ability's never actually been confirmed. This could be Greninja simply activating Protean; it's changed colours (black sides of head like Ash's hair, red stripe on head like Ash's hat) and shape (ever so slightly around the cheeks to be shaped like Ash's hair, with red zigzags for Ash's birthmarks on his cheeks) to adopt the traits of an individual whose strengths it wants to invoke (in this case - Ash's), just like how in the game it changes its appearance (by means of it's "type") when it invokes other types of moves. Everything else about it's design can be done by a normal Greninja. Frogadier already knows Cut, so the Water Swords are Greninja's version of that, and it could theoretically just form a Water Shuriken and somehow mount it on its back. On the Ash Greninja artwork beside (not in) the poster, it shows Greninja holding the Water Shuriken behind him, which shows it's not permanently affixed to or emitted by his back. So basically, this would be Ash's (eventual) Greninja activating Protean and becoming an "Ash-type", perhaps as its actions echo Ash's courage, etc.
...It'd be AWESOME if Ash were getting Mega Greninja, though. But this can serve as character develoment for Greninja whilst also leaving room for Ash to lose to Alain's Mega Charizard X initially before calling on Charizard Y to defeat it (since the calendar strongly implies Charizard Y will be appearing in December like it did with Talonflame, and then there's the Mega Charizard Y plushie). IF THE MEGA SCEPTILE ON THE POSTER IS ASH'S THOUGH, THEN XY JUST WON.
