I'm really hoping that when the 3rd game comes out (I think "Z" is all but confirmed at this point as a name to be honest) that they don't choose to do the 2 game split is my main concern. I didn't appreciate Black2 and White2, I liked all the singular titles Yellow/Crystal/Emerald/Platinum. I'm not really sure how they could do a split at all but it's been a worry since generation 5 for me that they would do it again. I'm not cynical enough to believe that they made 2 games in gen5 for a cash grab though - I could understand the concept behind it, but urgh. I like to collect all the games and it's just better if they compact it all into one game.
I am looking forward to playing Z though - I sort of glossed over ORAS a bit because I didn't think it metaphorically held enough water (ironically because the game was full of water, hehe) compared to XY. So an improved XY should be amazing :> I'm really intrigued by the Zygardes too. I'm wondering whether the Zygarde cell form will be classified in the Pokedex properly since the description of it says "scientists can't decide if it is actually a Pokemon" and I want to see if it will be in combat at all with the whole no moves thing - ie it would default to using struggle in combat. At present it's actually impossible to have a Pokemon with no moves, so there's all sort of implications about Cellgarde if he's actually combat available. Could be possible to lead to some glitches as well perhaps if they are going to enable it so even one Pokemon can have 0 moves. Despite everyone else thinking it's a tragic idea as well I'd... honestly kind of like to see extra Zygarde forms too (like for a 20% Zygarde, 30% and so on.) I think it would be interesting for it to be so versatile. And if that's not a thing they implement into the games I hope the community picks up on it as a concept and goes to have fun with all the Fakemons :)
Perfect Zygarde could be a danger though. One criticism I have is that they're getting a bit silly with the Base Stat Totals of the legends recently. He dodged the bullet of being a mega/primal at least, but I don't want to see Nintendo constantly putting stuff with a BST of 780 into the games (Mega Mewtwos, Mega Rayquaza, Primal Groudon, Primal Kyogre). He's described as being stronger than Yveltal and Xerneas which have 680, so I would speculate he'd be in the normal range of 700 BST which is standard for most legends if Nintendo are thinking with any sense, but considering they seem to be fixated on adding absurdly powerful things into the game I'm not so sure.
Honestly though I'm not sure what else they could add. They'll add an extra story arc with Zygarde I'm certain - possibly something with Yveltal and Xerneas goes wrong and there's a misbalance in their powers of life and destruction and Zygarde has to awaken his perfect form in order to balance it - since that's what his supposed power is. We didn't see much of Zygarde in XY to be honest so his power was never really shown to the audience. XY had such a wide range of Pokemon in the Pokedex though - it was HUGE and it was great that you got so much access to the old Pokemon, but I'm not sure it will feel as fresh with Z version unless they literally just decide to give you access to everything all in one game. I'd love to see some new megas (which I'm sure will happen) or maybe even some new Pokemon put into the 6th generation dex (not so sure will happen) to keep us on our feet.
My only regret right now is that players in Europe aren't getting a Pokemon game for Christmas. I sat through the whole of this year being like "when will they reveal Z? Come on!" because I wasn't a big fan of ORAS, and even though I think we're really close us Europes aren't getting a Pokemon game for the Christmas period (Not even Mystery dungeon) which is a little sad.