NeoGAF user botty noticed some things in Pokémon that seem to be pulled straight from Kingdom Hearts.
Here are his findings plus some other overwhelming evidence:
The sad thing is Kingdom Hearts mode is actually better than Pokémon's because you have to enter two modes (Super Training and Pokémon Amie) to do what Dream Drop Distance does in one.
The New Pokémon
A lot of the new Pokémon look like Dream Eaters.
The Elite Four/Hollow Bastion
After plowing your way through Victory Road you get to a fantastical castle. If you ever played Kingdom Hearts, you can't help but think about Hollow Bastion.
The Stained Glass Plus Other Plot Points
The battle versus Kalos Champion Diantha is a set-piece ripped straight from Kingdom Hearts. There is a Pokémon war just like the Keyblade war. AZ is a perfect misguided Hero/Villain like in Kingdom Hearts.