To everyone who says that there cannot be any new Pokémon or Megas in these games due to them being incompatible with XY as a result, let me point out that:
Platinum gave us new forms of Rotom, Giratina, and Shaymin, which weren't in Diamond and Pearl.
HeartGold/SoulSilver gave us Spiky-Eared Pichu.
Black 2/White 2 gave us Black and White Kyurem. And new forms for Thundurus/Tornadus/Landorus.
As for changed moves/abilities/etc.,
This has happened before. Hypnosis went from 70 accuracy to 60 accuracy in the jump from DP to Pt.
I, for one, think that Groudon/Kyogre may get new versions of Drought/Drizzle that allow for permaweather, since otherwise the storyline of Ruby/Sapphire is kinda ruined. I mean "OH NOEZ! TEAM MAGMA SUMMONED GROUDON AND WERE LOSIN ALL OUR WATER. HELP US!!!" kinda loses impact when you can just say "Oh, just wait a while, it'll fade eventually.".