Honestly, I was imagining Zygarde to shift in form every turn. Like, it takes him 4 turns to get to max form, kinda like Regigigas, except he does get more powerful each turn. It'll be an interesting mechanic, I'm not sure how it would work in programming but yeah. Or maybe he has a move that can transform him like Rayquaza.
I'm hoping Ash's Greninja means something for the games, but I won't be surprised if it ends up being just an anime one-off thing. Oh well. Either way, I'm hyped XD