Post-game spoilers: That's not post-game, I don't think. That's the main story. According to the main leaker from Something Awful, there isn't a post-game. You have Zygarde, and the Battle Tower type building (Battle Mansion, I think?), you have Mewtwo and a roaming Zapdos, and...that's pretty much it. He's found a few areas that SEEM like they could be used for something, but he can't figure out how to trigger anything there.
That's pretty much it as far as the post-game goes, as far as we know. If there's anything else, it's REALLY obscure.