I've come to the conclusion that I've limited myself to thinking based on generation 5 too much in regards to generation 6 speculation (in that Z-themed games would end generation 6 before we move on).
So let's take a few steps back.
The first Pokémon sequels we've seen, were Gold & Silver.
They took Generation I, its region, its Pokémon, its story, and expanded it all.
In other words, GS=Johto=Generation 2, did to RBY, what we've been expecting from XY-sequels to do to XY.
But there is nothing preventing Gamefreak from "finishing" generation 6 with the 7th generation. It may sound contradictory, but it would explain a lot of things.
It would explain, why we're starting to get reveals of Pokemon that are not part of the current generation with no game-announcement in sight.
It would explain why Zygarde is not the star of Movie 19, but its "story" is instead still being stretched out in the regular anime for months to come.
It would explain why they decided to develop ORAS first, instead of doing it the gen4-way.
It would explain why ORAS was made with the consideration of making all Pokemon obtainable between the games of X/Y/OR/AS (which is done for each generation as a whole).
It would explain why they're distributing ALL mythical Pokemon of all 6 generations one after another for the coming 8 months.
Pack your bags folks, we're going to the neighbouring region of Kalos. (I have a feeling it might be kind of industrial and german~)
A new game, in a new region (and Kalos), with a new generation (new Pokémon), that also happens to delve into the story behind Zygarde et al in the post-game and "ties up" all the loose ends of Gen6 "in a unique way".