There is a wildcard in regards to the X&Y-sequels that has been speculated about since the beginning of Generation 6 (when it became clear that we were indeed only getting ~70 new Pokémon in X&Y, lower than any generation before), that I haven't addressed in the previous posts: the possibility of NEW Pokémon being added mid-generation.
It's never been done before and it still seems unlikely (when trying to have realistic expectations, a third wave of Megas, Zygarde-formes and maybe some other form-changes would be what to expect in terms of new designs in XY-follow-up games), but the reveal of a new Pokémon today, Magiana, has very well re-ignited hope in this option, due to Magiana really being a new Pokémon that is not in the data of any generation 6 game, while the XY-follow-up has not even been announced yet.
I must mention however that there is a precedent for that, with Gen-4 Pokémon Munchlax having been revealed in May 2004, 2 months before Emerald, the final generation 3 game, was even announced for Japan (and more than 2 years before generation 4 actually started). So it could still be just a very early reveal of a Pokémon we won't see in a game for a while.
But it's too intriguing not to think about it. A bunch of new Pokémon added to the 6th generation in it's later part.. some people think of such hypothetical Pokémon as the 7th generation, but really, it would be a pseudo-generation. Undeniably intended as an expansion to what X&Y brought, it would still be the same generation, hence why I prefer to call it 6.5 in the title (it does start to beg the question, whether the concept of generations is even worth keeping around much longer after that. After all, if they plan to release Pokémon in smaller numbers, but more often, is it still necessary to group them like that?).
So, going along with that hypothetical, and the things previously speculated about the content of XY-sequels, we would perhaps explore much more than a few new places scattered throughout Kalos. I'm talking in terms of "Johto" being the expansion and sequel of Kanto. A new region directly southwards that is at least half as big as Kalos (with Kalos still being accessible in the game). Just like Johto (and Kalos itself actually), the size and diversity of the regional Pokedex is not a problem, since it would heavily rely on old Pokémon to fill most roles (encounter as well as plot-wise), with the fewer new Pokemon found only by those who look hard/long enough at each place.
There is no need for a new set of starters for this adventure, because:
-the Kalos starters were overshadowed by the Mega Kanto-starters in X&Y
-they can be made less repetitive by having Megas of their own introduced (a luxury the Unova starters didn't have for BW2, all they got was a few move-tutors)
-the amount of new pokemon is already lower than usual, and since this is still generation-6, using twice as many spots for starters would really be overkill.
An exciting and radical exception would be if they decided to spice up the "starters-concept" exclusively for the sequels, by trying something fresh, like making starters that are not of the traditional Grass/Fire/Water-formula (and then give you the Kalos starters mid-game anyway of course). Now that would be refreshing like nothing else, and it's a nice opportunity for that (since they can return to the traditional starters as soon as the actual 7th generation comes around).
There would obviously be more legendaries. People have been wondering about the absence of a traditional 580-base-stats trio for the first time in the series anyway, so if gen6.5 turned out to be the real deal, then that's one thing we could be farily certain about seeing. Along with more mythic Pokémon as the newly revealed Magiana or
...a legendary perfect stage of the Eternal Floette, as I speculated in the previous post.
And that brings me to the already revealed Zygarde formes, of which we are still unsure whether to consider the different stages, and the Zygarde cores, as separate species, or just Zygarde forms. In light of this possibility, the solution is straightforward, since Zygarde-Cores can be made a separate Pokemon, just like different stages in an evolutionary-line, or as in cases like Phione.
As for regular evolutions to old Pokémon, as much as I'd like seeing some (even if just to confirm that they haven't abandoned that idea... besides for the overdone eeveelution fanservice), I somehow doubt there would be any, as the Megas play a very similar role, while also being the gimmick of this generation, as well as part of the plot.
So in other words, the bulk would probably be Pokémon of the kind Gen-6 has started with (exotic type combinations and curious designs), but hasn't covered yet thematically, like
-desert-themed Pokemon (there is just a single Ground-type, Diggersby),
-mountains/caves (there is Noivern as the bat, but no native Fighters or Rock-monsters.. Rock is being abused for fossils too much as it is, give us more regular Rocks),
-some new faces in rivers (and while at it, make that fish a 3-stage with stats that will matter for a change again),
-more "generic Steel types" and machienry/Electrics,
-with the basic fairies done (3 pure lines and tons of retcons) maybe a few elemental ones too this turn,
-having added a big garden-Fairy that IS NOT Grass type, but no new actual plant-based Pokemon, it could use some of those (since it was just a goat and halloween-bros in that department)
-how about a bunch of proper pure Flying types to flesh the type out properly after 6 generations of avoiding it (especially with the whole Sky-battle gimmick going on somehwere far in the background...)
-perhaps a pseudo-legendary that is not part Dragon-type (Metagross and Tyranitar are lonely in this Dragonfest),