I feel like Toy Story would make a good eventual replacement for the "take-a-break-from-fighting-heartless" world of Winnie the Pooh. I wouldn't mind the 100 Acre Wood making one more appearance in the series though, so maybe Woody and co. get added for KH4? I just don't see Andy's Room being a very plausible normal world.
Also, ever since seeing the movie, I can't stop seeing a Monsters University world in KH3. Having Sora, Donald, and Goofy compete in the Scare Games (as opposed to the Olympic Games in the Coliseum) would be very different and exciting. I feel like a good plotline for this world could be Sora counseling Mike through his jealousy towards Sully for being the better scarer, since Riku is now a Keyblade master and Sora is not. Sora could explain to Mike that just because Sully is scarier does not mean he is more important, because they are all apart of the same team and share a common connection. I feel like Sully and Mike's relationship has some similarities to Riku and Sora's.