You know how Sora's rule to travel between worlds always forced him to hide from its inhabitants the existence of other worlds? Well, I believe that rule could end up disappearing in the future. I have found several hints of that during the story of Kingdom Hearts III. Honestly, I should be ashamed for not being able to realize this previously.   
  

1º) The first clue is in Monstropolis. You see, something that called my attention at the last moments of this world wasn't the fact that Sulley managed to hold Vanitas tight and "defeat him easily", but how he did it. I mean, they threw him through several doors so he couldn't return to their world easily. But there are 2 big problems here. First, considering Vanitas and other dark beings can use the Corridors of Darkness to go wherever they want, what could stop Vanitas from reappearing almost instantly with one of those dark portals?  

  
And second and most important, why did they throw him through several doors in the first place? All the doors in the factory are supposed to take you to children's rooms to scare them or make them laugh, except the special ones used to vanish some monsters, like the one used to vanish Mike and Sulley to the Himalaya alongside the Yeti in the first movie. The doors used here looked like the normal ones, which mean they throw Vanitas to the rooms of several children who could get really hurt by Vanitas. But then, I looked closely what we could appreciate through the doors, and I realized something crazy. Those doors didn't take you to children's rooms, but... OTHER DISNEY WORLDS THAT APPEAR IN THE VIDEOGAME!!!  
  
If you record the scene, and you stop it at the right second, you can appreciate it. Check:  
+ The first door, the red one, through which Sulley throw Vanitas, if you look closely, you can recognize the snowed forest of Arendelle by night! Just the are where you are after the first cinematic of the world, before start climbing!  
+ The second door, the green one, if you check the floor at the other side of the door, you'll realize is the same floor that you can find at the city of Corona's Kingdom, where you can play the dance minigame!  
+ The third door, made of wooden boards, it takes to an alley of a big modern city at night. This city can't be other than... San Fransokyo!  
+ The last one in not really clear, but judging by the colors of the place, I would dare to say that it could be some zone from Galaxy Toys, from the Toy Box's world!  
  
An error, a joke, or another reason? If Montropolis know about the xistence of other worlds, the same ones Sora has visited in the past, it means the whole rule about keeping "the world's order" could be losing sense for the future. Could we end up seeing Disney characters from one world appearing in another one? Tha sounds really tempting!!  
  
2º) San Fransokyo is another world who could technically know about the existence of OTHER worlds beyond his own. Remember, Hiro told Sora's team the story about the original Baymax; about how it was lost "into another dimension". Honestly, the differences between "dimension" and "world" are very thin in some cases. Not to mention the fact that, when Hiro and Baymax entered into the portal in the first movie, the place where they entered and rescued Abigail looked quite a lot like "The Ocean In-Between". This world could be a few steps of breaking the walls of its own reality. Imagine if Big Hero 6 travel to other Disney Worlds.   
  
3º) The whole thing about dividing the world into two at the Toy Box literally opened a gate for the characters of Toy Story. If Buzz's laser turned real, then we could end up visiting the world of the REAL Buzz Lightyear, based on Disney Channel's animated series "Buzz Lightyear of Star Command". That would be epic!!  
  
4º) Between the details revealed in the Ultimania book of Kingdom Hearts III, it was mentioned that Tetsuya Nomura loves Hades because he finds him as an interesting character. Also, he wants to see where it goes next. Well, I think I have a clue about it. Just like Maleficent in the original game, I believe Hades could become the new leader of an alliance between Disney's Villains. Remember, unlike other Disney Villains, Hades is a God, he's immortal, so there's no secure way to get rid of him permanently. Many of the other Disney Villains were defeated/destroyed and they are, technically, dead. Hades is the Lord of the Dead, so it would be logical if those villains end up in the Underworld and Hades recruits them. This is basically what happened in Hercules' tv series, "Hercules and the Arabian Nights", where we had a crossover between Hercules and Aladdin's tv series when Jafar appeared in the Underworld. (Which is quite strange because Jafar is not greek).   
Still, it could be an interesting new role for Hades. Which enemies could he recruit? Well, Maleficent resurrected, but she could still join forces with Hades. Which Disney Villains are dead? We have Jafar, Ursula, Oogie Boogie, Scar, Clayton (if they could use Tarzan again), Shan Yu, the MCP (althoguh this one is a machine), the Evil Queen (Snow White), Lady Tremaine and her daughters (They denied Anastasia' redemption), Claude Frollo, CLU (another machine), Gaston (Sora still hasn't meet him), Mother Gothel and Davy Jones. Hans didn't reappeared after defeating the Heartless Skoll, but I doubt he's dead. He was still alive at the end of "Frozen".

 

  
Well, what do you think?

not to mention twilight town, they know you come and go. it would be an interesting idea but unless some characters are shocked or puts some in danger then it doesn't matter

1 hour ago, teh lazy prince Xylek said:

not to mention twilight town, they know you come and go. it would be an interesting idea but unless some characters are shocked or puts some in danger then it doesn't matter

Well, how about a remake or certain crossover?  
  

In "Hercules and the Arabian Nights", both Jafar and Hades tried to defeat each other's nemesis, and they both failed. Then, they had the idea of making both heroes fight between them, in hopes they will destroy each other. Of course, they needed an excuse, so they kidnapped each hero's best friend (Abu for Aladdin and Icarus for Hercules) and framed the other hero. After some fighting, both heroes realized they were tricked and join forces to rescue their friends. They win and leave in good terms with each other.   

  
it was actually a good plot, except for the fact it was the slim Hercules against the already married Aladdin; so it clearly could have been improved. In my version, Hercules and Megara are already married after Hercules' movie. As a consequence, Hades and Jafar take their attacks to the other hero more seriously. They fail, so they plot the set-up kidnapping their enemies wifes (Jasmine for Aladdin and Megara for Hercules). Heroes fight in Agrabah, where Hercules' reputation as a hero can't interfere. There's a long fight, we also have some facing between Pegasus and Carpet, as well as Phil and Genie. They finally realize the set-up and join forces to save their wives. By their part, Jasmine and Megara manage to free themselves and take some action by themselves. After the heroes defeat Jafar, Jasmine and Megara trick Hades into falling to the Stygian River (like in Hercules' movie). Both heroes leave in good terms.  
  
If some characters from one Disney World appear in a different, there will be definitely shock and endanger. Naturally, to avoid certain "absurdities", some characters should appear in worlds with a little similarity to their own. For example, Ariel appearing in Neverland, or Simba in Tarzan's jungle (if they could use it again).

 

  
Well, what do you think?

1 hour ago, Ursalink said:

Well, how about a remake or certain crossover?  
  

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In "Hercules and the Arabian Nights", both Jafar and Hades tried to defeat each other's nemesis, and they both failed. Then, they had the idea of making both heroes fight between them, in hopes they will destroy each other. Of course, they needed an excuse, so they kidnapped each hero's best friend (Abu for Aladdin and Icarus for Hercules) and framed the other hero. After some fighting, both heroes realized they were tricked and join forces to rescue their friends. They win and leave in good terms with each other.   

  
it was actually a good plot, except for the fact it was the slim Hercules against the already married Aladdin; so it clearly could have been improved. In my version, Hercules and Megara are already married after Hercules' movie. As a consequence, Hades and Jafar take their attacks to the other hero more seriously. They fail, so they plot the set-up kidnapping their enemies wifes (Jasmine for Aladdin and Megara for Hercules). Heroes fight in Agrabah, where Hercules' reputation as a hero can't interfere. There's a long fight, we also have some facing between Pegasus and Carpet, as well as Phil and Genie. They finally realize the set-up and join forces to save their wives. By their part, Jasmine and Megara manage to free themselves and take some action by themselves. After the heroes defeat Jafar, Jasmine and Megara trick Hades into falling to the Stygian River (like in Hercules' movie). Both heroes leave in good terms.  
  
If some characters from one Disney World appear in a different, there will be definitely shock and endanger. Naturally, to avoid certain "absurdities", some characters should appear in worlds with a little similarity to their own. For example, Ariel appearing in Neverland, or Simba in Tarzan's jungle (if they could use it again).

 

  
Well, what do you think?

I like it but there would be need to be some sort of threat like the worlds falling to darkness like in Kingdom Hearts 1. 

I mean we can bring back King Triton.  We never got a clarification as to why he detested seeing the keyblade and why he warned Sora and his buddies not to interfere in other worlds.  We could have a crossover with The Little Mermaid 2.  I think Nomura could improve upon the undesirable Disney sequel.  And since Ariel is no longer 16 years and approximately twelve or thirteen years have passed in her and Eric's world, they could forget about meeting Sora, Donald and Goofy and just remember the movie's events plus Morgana, Ursula's "crazy" sister almost feeding their infant daughter Melody to her shark minion but being stopped by Triton.

Well, like I said, Hades becomes the new leader of Disney Villains (like Maleficent was in the first game), and tries to conquer the cosmos. Maybe Hades "recycles" a plot left by Xehanort or something.   

  
Anyway, there's a big timeskip between The Little Mermaid and its sequel; and obviously, they can't age Sora so abruptaly like that. But there's one alternative. To defeat Master Xehanort once and for all, they do it using the fact that he himself is a portal through time and space. Well, how about if, by accident, once he defeated Xehanort, Sora "inherited" part of his powers to travel through time? That could be an excellent resource for future stories. Sora could avoid aging too much and face Disney stories that skip several years. Also, I believe Sora could have another use to such power if, somehow, finds a way to bring back the Keyblade Weilders from the past (the ones from KH:UX) and stop whatever is what the Master of Masters is planning.