Depends on how far they are with the next KH game in terms of development. For KH2 they started development after the release of KH: Final Mix in December of 2002 and took 2 1/2 years to make, Dream Drop Distance began development in Early 2010 and lasted until late 2011 for development, KH3 started development around late 2012/early 2013 under the luminous engine before development restarted in the Unreal Engine 4 in October 2014 which lasted a solid 4 1/4 years to make.
Now that they are utilizing the Unreal Engine 4 they can still reuse the same character models and maps from KH3 as future reference and can get the development done faster for the next games, and I expect the next one to come out by 2021 or 2022 (KH 20th Anniversary) with KH4 by 2023/2024.
Regarding Disney worlds for the next game and KH4 this is my envisioned listing:
New KH game before KHIV:
- Wreck-It Ralph
- Meet the Robinsons
- Coco
- Treasure Planet
- Atlantis
- Dumbo
- Pocahontas
- Nutcracker and the Four Realms or Oz the Great and Powerful
- Inside Out
- Mickey and the Beanstalk
- Onward
- Moana
- Robin Hood
Kingdom Hearts IV:
- Zootopia
- Raya and the Lost Dragon
- Frozen II (returning world)
- Toy Story (returning world)
- Princess and the Frog
- The Incredibles
- Aladdin (returning world)
- Lilo & Stitch
- Jungle Cruise
- Brave
- Winnie the Pooh (returning world)