From what I've been gathering, the theme for KH4 revolves around death and rebirth. With Sora in Quadratum and the reveal of Strelitzia residing there, I think this is a good guess. With that in mind, here's the worlds that I think and hope could fit in.
A Galaxy Far, Far Away (Star Wars): while I'm a huge skeptic of including this at all since it was an acquired property, I think the clues we have are pointing to its inclusion as a certainty. The KH4 trailer dropped hints that more realistic worlds will be included versus the cartoon aesthetic. Since the Star Wars series revolves around the struggle between the light and dark and has the concept of Force ghosts persisting after death, Sora could learn from them and use this knowledge to return to the Realm of Light. If we're also going to expand on the size of the worlds, limiting it to one planet in the franchise could be fixed on adding space travel to the mix. Obviously Darth Vader would be a Disney boss and the party member would be Luke Skywalker.
City of Witches (Hocus Pocus): I could only hope and pray that they would consider this as an option. You can 100% adapt the plot of the film into something related to Kingdom Hearts: rather than collect life force, the Sanderson Sisters would need to collect hearts from children in order to remain alive and young forever. We can even play on Sora's internal darkness and have him be convinced by the Sisters that stealing hearts could help him return to the Realm of Light, only to have character development which leads to another Disney boss fight. Due to the movie's insane popularity even 30 years after its release, people would absolutely love fighting the sister trio. Your party member would be Billy Butcherson, the zombie ex-lover of Winifred.
Land of the Dead (Coco): though this is a cartoon world, I can't imagine this not making it in with the theme of life and death. This would also follow the plot of the film, with Sora teaming up with the Disney characters to learn how to return to the realm of the living. He can use this experience to learn a way to get back to the Realm of Light at Quadratum. The world for this would be so brilliantly colored and has the potential to be massive. The Disney boss could be Ernesto de la Cruz, though I feel a heartless based on the guardians would fit as well. Your party member would be Héctor since I feel Miguel wouldn't fit as a good party member.
Motunui (Moana): with the success of The Caribbean in KH3, this could be a proper expansion on the concept of seafaring. Within this world those who have passed on take the form of an animal spirit, so Sora's experience could be learning about that process and believing that Te Fiti would provide him with answers. This would be a great opportunity to have two party members in both Moana and Maui since both have fighting capabilities. Te Fiti could also serve as an amazing boss fight when she is in her form as Te Kā. Also, with hearts in this world having physical manifestations it could further build on the Kingdom Hearts lore, possibly solving a mystery as to how a heart can be separated from its body and not create a Heartless.
Atlantis (Atlantis: The Lost Empire): This world is a LONG time coming. Similar to Motunui, passing on in this world is not the end of the everything, at least for a select few. In fact, this world also has hearts with a physical manifestation, with this one appearing as the Heart of Atlantis. This world could be a replacement for Motunui or work in conjunction with it, building off of each other as Sora figures out how to return to the Realm of Light. The Disney bosses would be Rourke and Helga, possibly using a corrupted Ketak or Leviathan Heartless. Your party member would be Kida, not Milo, since she is more adept at fighting.
So those are some examples of what I think could fit well into the universe in a seamless way! Since each iteration brings something out of left field, I could even see them not ruling out their television series or even Disney Channel Original Movies. If we continue the New Seven Hearts at all, here's some candidates I think they could, and honestly should, go:
Mary Poppins (Mary Poppins) - we NEED to incorporate her in some sense. It would be so cool.
Gisele (Enchanted) - her situation is the same as Mary Poppins, a person popping to and from an animated world, but I'd like to see this film represented anyways.
Tiana (The Princess and The Frog) - she needs her time in the spotlight, that's it.
That's it! Let me know if y'all think this would be fun!
A Galaxy Far, Far Away (Star Wars): while I'm a huge skeptic of including this at all since it was an acquired property, I think the clues we have are pointing to its inclusion as a certainty. The KH4 trailer dropped hints that more realistic worlds will be included versus the cartoon aesthetic. Since the Star Wars series revolves around the struggle between the light and dark and has the concept of Force ghosts persisting after death, Sora could learn from them and use this knowledge to return to the Realm of Light. If we're also going to expand on the size of the worlds, limiting it to one planet in the franchise could be fixed on adding space travel to the mix. Obviously Darth Vader would be a Disney boss and the party member would be Luke Skywalker.
City of Witches (Hocus Pocus): I could only hope and pray that they would consider this as an option. You can 100% adapt the plot of the film into something related to Kingdom Hearts: rather than collect life force, the Sanderson Sisters would need to collect hearts from children in order to remain alive and young forever. We can even play on Sora's internal darkness and have him be convinced by the Sisters that stealing hearts could help him return to the Realm of Light, only to have character development which leads to another Disney boss fight. Due to the movie's insane popularity even 30 years after its release, people would absolutely love fighting the sister trio. Your party member would be Billy Butcherson, the zombie ex-lover of Winifred.
Land of the Dead (Coco): though this is a cartoon world, I can't imagine this not making it in with the theme of life and death. This would also follow the plot of the film, with Sora teaming up with the Disney characters to learn how to return to the realm of the living. He can use this experience to learn a way to get back to the Realm of Light at Quadratum. The world for this would be so brilliantly colored and has the potential to be massive. The Disney boss could be Ernesto de la Cruz, though I feel a heartless based on the guardians would fit as well. Your party member would be Héctor since I feel Miguel wouldn't fit as a good party member.
Motunui (Moana): with the success of The Caribbean in KH3, this could be a proper expansion on the concept of seafaring. Within this world those who have passed on take the form of an animal spirit, so Sora's experience could be learning about that process and believing that Te Fiti would provide him with answers. This would be a great opportunity to have two party members in both Moana and Maui since both have fighting capabilities. Te Fiti could also serve as an amazing boss fight when she is in her form as Te Kā. Also, with hearts in this world having physical manifestations it could further build on the Kingdom Hearts lore, possibly solving a mystery as to how a heart can be separated from its body and not create a Heartless.
Atlantis (Atlantis: The Lost Empire): This world is a LONG time coming. Similar to Motunui, passing on in this world is not the end of the everything, at least for a select few. In fact, this world also has hearts with a physical manifestation, with this one appearing as the Heart of Atlantis. This world could be a replacement for Motunui or work in conjunction with it, building off of each other as Sora figures out how to return to the Realm of Light. The Disney bosses would be Rourke and Helga, possibly using a corrupted Ketak or Leviathan Heartless. Your party member would be Kida, not Milo, since she is more adept at fighting.
So those are some examples of what I think could fit well into the universe in a seamless way! Since each iteration brings something out of left field, I could even see them not ruling out their television series or even Disney Channel Original Movies. If we continue the New Seven Hearts at all, here's some candidates I think they could, and honestly should, go:
Mary Poppins (Mary Poppins) - we NEED to incorporate her in some sense. It would be so cool.
Gisele (Enchanted) - her situation is the same as Mary Poppins, a person popping to and from an animated world, but I'd like to see this film represented anyways.
Tiana (The Princess and The Frog) - she needs her time in the spotlight, that's it.
That's it! Let me know if y'all think this would be fun!