So here are my predictions about the basics of what it will be about. I tried to be as realistic and accurate as possible with the information, official and unofficial, that is available so far.
(this should be self-explanatory)
- Zygarde as the mascot
The 2 forms will be meant to counter (at least plot-wise) Xerneas or Yveltal (who might get a new form, Mega, Primal or similar as well), depending on which one wasn't woken by Flare in the initial Version (X2=ZygardeX vs Yveltal, Y2=ZygardeY vs Xerneas). This quite specific prediction is implied by:
- 2 games, 2 Zygarde forms
- hackers having found that XY/ORAS are programmed to be compatible with 2 more games (the pentagon symbol has 6 origin-region values, here is the source: pastebin.com/zPgwx6mb). Unfortunately for any Kanto-fans, this pretty much confirms that we aren't getting any more cash-milking nostalgia remake-business games ... for now.
- the 2 hidden moves "Thousand Arrows/Thousand Waves", as these are identical to Zygardes current only signature move Land's Wrath (all 90P spread-Ground moves) with an additional effect each (hitting airborne targets and trapping enemies respectively), not to mention the new moves have animations involving green hexagons flying across the screen. (It pretty much gives away that Land's Wrath will be like Kyurems Glaciate. It will change to one of the other moves when it changes form, Glaciate->Ice Burn/Freeze Shock, Land's Wrath->Thousand Arrows/Thousand Waves)
if they're simply following generation 5s follow-up games (since Zygarde seems to follow Kyurems case almost too closely) they might just slap a 2 to each Version, resulting in X2 & Y2. However Gamefreak could cleverly make use of the fact that the letter Z resembles the number 2, and call them XZ and YZ. Not to mention Zygarde itself might somewhat resemble Xerneas/Yveltal in its design.
- The Versions might be called XZ & YZ
- Third round of Mega evolutions
The only ones I am confident enough to write down about getting Megas, are the Kalos starters. Because GF would be stupid not to grab this opportunity to make the "repetition" of the newest starters in the sequel more interesting with a mechanic this generation introduced in the first place!
- Battle Frontier!
a whole new part of the region will be explorable (likely even including the starting town), as well as a handful of new places, routes and dungeons popping up throughout the rest of Kalos.
- Southern Kalos
This isn't merely self-explanatory from looking how the previous generation did this, this time there was plenty of places in XY already which felt unused or not fully explored, to imply that this is the case (Rhyhorn Race-tracks, all those locked Power-plants, Train stations..). And of course there is Kiloude City which is supposedly an example of southern-Kalos culture.
Of course the path through the region (and some of the gyms, their types and leaders etc) will be radically different as there will be new protagonists and the story taking place years after XY, with a climax in the mascot-involved plot mentioned above.
- Changes in Kalos
The wardrobe from XY will definitely make a return for Kalos-sequels at least, be adjusted to fit the new characters, as well as get expanded.
- References to XY and ORAS
There should be plenty of familiar faces and references, obviously to XY with these being their sequels, but likely also some from ORAS. In particular, what's going on with Looker at the end of ORAS should be explained, as will probably AZs background (and his special Floette might after all be made available somehow, since it has unique stats compared to a regular Floette and a signatrue move called Light of Ruin). But maybe we could see even less expected ones, like Zinnia or Emerald-exclusive gym-leader Juan.
Going any further from here is pure speculation though, so I will stop here.