Gonna take another route of speculation that has crossed my mind...
I'm wondering if we should start taking the Strange Souvenir into serious consideration with the reveal of Magearna. I'm not saying that the SS depicts Magearna, but what if those two things are a sign of Gen 7 coming sooner than we thought? If that's the case, then I'm really hoping it follows the Kanto/Johto scenario and then we would still get our Zygarde-centric story that would focus on both the new Gen 7 region and Kalos. Programming-wise, I think it may be doable; they were able to fit Johto and Kanto into HGSS on the DS. The 3DS I imagine has quite a bit more memory space in it than the DS has(don't know if I'm using the right terms, but yeah), so I think it MIGHT be possible to cram Kalos and a Gen 7 region into the new game.
Ahh, BUT... what if we get it on the NX instead? Kinda doubting that one because we don't even have a release date for it. They're supposed to talk about it during E3(I almost said E4, lmao) this year, so the speculation will get much more interesting if we don't hear about the new game by then.