Okay, so everyone's been doing this sort of thing, so I thought I'd take my little shot at it. Bear with me, I tend to be bad at wording things, so some of what I say might not make a lot of sense.
So, we can basically be sure that Zygarde is the Z legendary to the X and Y of Xerneas and Yveltal. We also can be sure, based on its deficient stats and movepool, that it will receive at least one new forme. But what precisely will Pokemon Z be?
I see a few possibilities. Firstly, Pokemon Z could just be a single game that's basically what Emerald was with Ruby and Sapphire. Same basic plot as X and Y, a few new areas, Zygarde directly involved, both legendaries awakened. I very strongly doubt this will happen though.
Did X and Y give anyone else the feeling of being intense but unfinished? I got that sort of feeling from them. There of course are very few new Pokemon, less even than were introduced in the Johto region. There's Kiloude City, a major city unconnected to the rest of the region despite still being on the same landmass. There's a puzzling sparsity of legendaries. This leads me to believe that Z will be a sequel. But branching off of this, I see two possibilities for sequels, one much more likely than the other.
The first possibility is a sequel in the vein of Black 2 and White 2. Same region, same starters, new areas, no new Pokemon. While Black 2 and White 2 could be argued to have basically been Generation 5.5 due to them having been indisputably 5th gen games but taking place further down the timeline and continuing off the previous plot, they weren't really much new material. Don't misunderstand me; they definitely tied up a lot of loose ends to the ending of the story of Black and White, they definitely built off the ending we were given, and they were definitely among my favorite games of all time. But in spite of all this, it was largely just Black and White with a time skip. So this is one possibility for Pokemon Z. The Kalos region a couple of years later, with vestigial factions of Team Flare attempting to unearth Zygarde for some purpose that frankly I can't think of. A few new areas, but deniably part of the Kalos region, and no new Pokemon, but some new formes, namely one or two (I'm leaning towards one) for Zygarde.
The second possibility I find much more likely. In this possibility, we either get a whole new nearby region or a very large addition to the Kalos region to the point where it could nearly qualify as an additional region. There would be access to the Kalos region we know but we'd start in the new region or area (akin to Black 2 and White 2, but it will differ). There would be a number of new Pokemon. If it's merely a new area, than probably a small number and not necessarily even new starters. If it's a complete new region, then potentially a full set of new Pokemon (though likely small due to it still being Generation 6), with new starters, new legendaries (but still of course focus on Zygarde), you name it. It'd still be Generation 6, and basically would just be adding onto the awkwardly brief and very lacking (look at how little type diversity there is) Generation 6 PokeDex. Perhaps a lingering faction of Team Flare would awaken another new legendary tied to the Ultimate Weapon (a new legendary hailing from the new large area/region), and the extreme destruction it would threaten the regional ecosystem with would lead to the awakening of Zygarde. This could be a good indicator for a new Poison type legendary, which we do not yet have other than Toxic Plate Arceus. What would this new region or area be? If it's just a large new area, we can assume it'll be Southern Kalos. Southern Kalos was referenced a few times in X and Y, and of course Kiloude City would finally be able to be connected through means other than a train. The Kalos region we currently have really only adapts the northern two-thirds of France. Southern Kalos would be the remainder, possibly with Andorra, Monaco, and parts of Switzerland and Italy. If it's a distinct new region, based on the available in-game information, we can determine it'll be based on either just England, just the island of Great Britain, or the British Isles as a whole. There are many references to wars between Kalos and another neighboring region. There's a reference to rivalry between the two regions. Kalos is mentioned to have historical ties to Unova. Of course, there were a number of wars between France and England. France and England are notorious for a long fierce rivalry. And of course France aided the 13 Colonies in the Revolutionary War. Though I do have my own region based on the British Isles which I would have to heavily revise if this proves true, I honestly greatly hope that it does prove to be this theory that's correct.
Finally, the new Pokemon additions. If Z just brings in a few new areas, there likely won't be any additions except for some Mega Evolutions. If it's just Southern Kalos, probably it'll be a new addition to the Kalos Dex of roughly equal size to the other three parts, introducing a roughly equal number of new Pokemon mixed with a roughly equal number of older ones. If it's a new region, there most likely will be an entire new PokeDex, though probably a relatively small one. This would include new starters, a number of new generic Pokemon, possibly some true evolutions (not Mega Evolutions) of existent Pokemon (hopefully some of Kalos Pokemon since a lot of them seem very unfinished and very capable of further evolution), and a couple of new legendaries, including the possible Poison type one I mentioned earlier. Also, the Poison type likely would have a type combination that'd make it strong against both Xerneas and Yveltal, so possibly a Poison/Electric, Poison/Rock, Poison/Ice, or Poison/Fairy type? But it could just as easily be pure Poison. The way I see it, the maximum number of new Pokemon in this potential new region would be just enough to bring the total number of Generation 6 Pokemon to 190. That's just an estimate of course, but an estimate based on the number of Pokemon originally intended for Generation 1. Remember, this is just a maximum. It's more likely it'll be fewer Pokemon.
Anyway, that's what I have to say about Pokemon Z. I might do something similar on the topic of Pokemon Ruby and Sapphire remakes, since this was kind of fun.