"Zygarde monitors the Pokémon ecosystem to ensure that it’s not disrupted. Zygarde believes it has a duty to protect the ecosystem. While the power it possesses is overwhelming, this Forme contains only 50% of Zygarde’s cells. OUT IN THE WORLD, AN EQUAL AMOUNT OF POWER WAITS TO BE BROUGHT TOGETHER"

If this isnt the biggest hint for Kalos in the post game then i dont know what is.

>inb4 "that means they want you to trade Zygrade from XY". It wouldnt make sense considering how much they promote Zygarde complete forme. They didnt ask you to trade Pokemon from old games to get special features of them in the current game (for example the didnt ask you to send Mewtwo from old games to XY to obtain mega Mewtwo)

http://www.pokemon-sunmoon.com/en-au/pokemon/zygarde-50-forme/