if you can pick one world...

what anime do you want in kingdom hearts? I think bleach would work best; imagine fighting heartless and hollows.

 

Yu-Gi-Oh. You could fight Heartless by duelling them.

 

I'd love it if Fullmetal Alchemist Brotherhood could be a part of Kingdom Hearts, but it's way too mature and graphic, but if they toned it down and reduced the complexity of the plot then it'd be awesome. Aside from that, I would love to see Naruto and Sora together, same as with Sasuke and Riku. It could be about Sasuke surrendering his heart to darkness and then Riku gets him out of it through sharing his own experience, assuming this is Kingdom Hearts III we're talking about and hopefully Riku plays and ever-present role in it, like a constant option as a party member I suppose.

 

If there were any anime in the world that I would incorporate Kingdom Hearts with, it would be Fairy Tail. They have similar concepts, the whole "level up" system, and Friendship conquers everything.