In Descendants 1, Mal explains that “all of the kingdoms united to create the ‘The United States of Auradon’”
But she said “Beast united all the kingdoms to create the United States of Auradon.” I doesn't seem like there was anything Kingdom Hearts related connected to that. But she lived in the isle, remember? Maybe they didn’t get the full story there. Plus, Beast probably was the one who united all the worlds into one country, despite still not actually merging the worlds together. 
This also explains why the fairy tale characters have modern technology. Characters from Sleeping Beauty and Cinderella, for example didn’t have that technology, but characters from Big Hero 6, and Toy Story do. When the worlds combined, the fairy tale characters were given these things they have never seen from other worlds. 

In the Kingdom Hearts X saga, it is explained that in ancient times, all the worlds were once one huge mega-world.

Well, what if “worlds” and “kingdoms” mean the same thing?
I think that the perfect way for the Kingdom Hearts franchise to end, is for the end to be the same as the beginning. 
All of the worlds were one in the beginning, so they will all be one in the end. So in the opening for Descendants I, Mal would’ve been summarizing the end of the last Kingdom Hearts game. 

Now you may be wondering: What happened to the villains? They died in Kingdom Hearts, right? A lot of them have, but the ones that appeared in Decendants all have some sort of way they might have survived.

Maleficent: Her fate is not yet revealed. In fact she died twice. First in Sleeping Beauty, she was slayed by Prince Phillip. Second in Kingdom Hearts one, killed by Riku. She came back to life both times, so even if she is defeated “for good” in a future Kingdom Hearts game, she’ll probably find a way to come back. 
Jafar: He never died, he was only turned into a genie. If he is freed by a master, he could turn human again.
Cruella De Ville: She never died.
The Evil Queen (Regina): In Snow White And The Seven Dwarves, she was supposedly killed by a boulder falling on her. But, we never seen the boulder hit her, so she could have escaped. 
Ursula: In The Little Mermaid, she was stabbed in the heart. But the thing is, octopuses have 3 hearts. So Ursula could survive off of her other two hearts. 
Gaston: He fell into that huge pit. But we never saw him hit the ground. Who knows what really happened to him?
Captain Hook: He was eaten by the crocodile. But in every Peter Pan sequel, Hook is always there. It is neverland after all, no one can die there.
Dr. Facilier: He was dragged into some kind of other realm. He never died. And when all the worlds became one, he was brought to the real world once again… just not a great part of it. 
Hades: In the movie Hercules, he was trapped in the river of souls. But that movie takes place years before Kingdom Hearts one. In that game, and almost every one after it, Hades was there. You can’t kill a god. 

So what do you think? Do you think it’s possible? I don’t think there is anything proving it wrong. Since the Descendants series is all over now, the only thing that can contradict the theory now is a future Kingdom Hearts game. I hope that doesn't happen…  

While I don't think it'll happen (Nomura is a crazy genius that makes stuff as he goes along and might have the same idea as you if he gets to thinking about it, but I doubt Disney will allow it), I'm gonna accept it as my headcanon until the games disprove it!! ^-^

1 hour ago, Moogle13 said:

While I don't think it'll happen (Nomura is a crazy genius that makes stuff as he goes along and might have the same idea as you if he gets to thinking about it, but I doubt Disney will allow it), I'm gonna accept it as my headcanon until the games disprove it!! ^-^

Why wouldn't they allow it? They let Nomura use stuff from other movies, why not this? And it's not like we would even get a world, it could just be one scene hinting at it.

It's unlikely that it'll happen because it's a big connection, and Disney is VERY protective of their properties. I mean, they could hint at it, but nothing really big will probably ever be confirmed. It just doesn't seem like it would be along the lines of something Disney would want - it's different from using Disney as worlds, it's connecting two entire franchises and giving SE a lot of power. If Sora can't get into Smash Bros because of Disney, I doubt they'll let Square do this.

On 12/12/2020 at 6:25 PM, Moogle13 said:

It's unlikely that it'll happen because it's a big connection, and Disney is VERY protective of their properties. I mean, they could hint at it, but nothing really big will probably ever be confirmed. It just doesn't seem like it would be along the lines of something Disney would want - it's different from using Disney as worlds, it's connecting two entire franchises and giving SE a lot of power. If Sora can't get into Smash Bros because of Disney, I doubt they'll let Square do this.

Well, Disney never actually said anything about Smash before.

It could be possible. Let's look at the concept as a whole.

It would make sense I theory it would explained how  the Beast knows anything about Maleficent, Jafar, The Evil Queen, Cruella De Vil, and Hades even though he never met them.

But we have to take a few things in hyperthetical questions.

1. If the end result of Kingdom Hearts leads to Descendants then when did it happen?

2. Who murged the world's together as one in the first place.

3. Why did that person do it?

4. Wear are the Heartless, Nobodies and unversed and don't say there all on Isle of the Lost believe it or not guys that island isn't that big and even still they can escape easily and since darkness would exist in people's heart we would see them all over auradon Prep give Audrey's presence.

5. What happened to the door to light is it still around?

6. Would it be risky because uniting all the world's as one would mean all seven princess of heart are in this one world making it easier to catch.

7. How come we don't see any keyblade weilders.

Um Sora did make it in to smash.

On 12/12/2020 at 5:25 PM, Moogle13 said:

It's unlikely that it'll happen because it's a big connection, and Disney is VERY protective of their properties. I mean, they could hint at it, but nothing really big will probably ever be confirmed. It just doesn't seem like it would be along the lines of something Disney would want - it's different from using Disney as worlds, it's connecting two entire franchises and giving SE a lot of power. If Sora can't get into Smash Bros because of Disney, I doubt they'll let Square do this.

 

On 12/12/2020 at 5:25 PM, Moogle13 said:

It's unlikely that it'll happen because it's a big connection, and Disney is VERY protective of their properties. I mean, they could hint at it, but nothing really big will probably ever be confirmed. It just doesn't seem like it would be along the lines of something Disney would want - it's different from using Disney as worlds, it's connecting two entire franchises and giving SE a lot of power. If Sora can't get into Smash Bros because of Disney, I doubt they'll let Square do this.

Um Sora did make it in Smash Bros. Also if it does happen now is possible since Disney is making Kingdom Hearts a Priority