New:
Pocahantes
Toy Story
Finding Nemo
Old:
Halloween Town
Neverland
The Land of Dragons
Get Rid Of:
Agrabah
Monstro
Wonderland (I don't dislike it though)
I'm actually one of the few who are totally fine with Atlantica. I enjoyed its openness in KH1, and it served as quite a fun sidequest world in KH2, so I don't mind too much. I prefer how it was handled in KH1, but I'll take what I can get. I just really love The Little Mermaid as a movie. My only complaint would be how horribly Ursula's revival was handled in KH2. It felt so incredibly random. Still, if KH3 could find a way to incorporate Finding Nemo, I would be okay with saying goodbye to Atlantica.
Same goes for Olympus Coliseum. While I admit it felt overused and a bit boring in Birth by Sleep, coded, and (to an extent) Days, it was handled awesome in KH1 and decent in KH2. I've always loved the Cup tournaments, and they really need to bring those back.
As for non-Disney worlds, I hope KH3 has access to all three of the "hubs" of Traverse Town, Twilight Town, and Hollow Bastion/Radiant Garden. Twilight Town is quite possiblity my favourite world in the series and I adore Traverse Town and Radiant Garden as well. I also wouldn't mind a new hub town though either.