The implication right now seems to be a game starring Young Xehanort & his trip around the world(s). In truth, this is uncertain, we could be playing as anyone at any point in time, but the project name & teased art plus the new scene with YX in RE:Mind points to him traveling "the world" before his graduation, so that's the most likely scenario.
This puts the game 75 years before KH3, or 64 years before BBS. It could be possible for PX to resort to the good ol' illusion/data worlds cop-out, MoM is there so the Book of Prophesies could very well come up. However, the dialogue in the RE:Mind scene implies that YX went to real worlds & met real people. Afterall, he was trying out the black coat, which wouldn't be needed if he was just visiting an illusion.
So then, real Disney worlds 75 years before the present point in the timeline, what even works for that? The gap in time seems too great for most Disney characters to stay the same age in both points in time. Yes, it's been established that time moves at different rates in different worlds, but even then this would be seriously stretching it. Even if the canon technically checks out, the sight of a teen who becomes an old man while everyone around him stays the same age would just wouldn't feel right. There's little real reason to have such direct dissonance between Disney & original characters on screen. So it may seem that most worlds that have already appeared in the series are ruled out.
The next possibility then is to go all-in on Disney properties that have not yet been featured in the series. Be it Jungle Book, or Robin Hood, or Zootopia to give examples. This would be rather unusual given that new worlds tend to be reserved for bigger scale games. Sure UX has Wreck-it Ralph, but that's more the lone exception than the rule. Furthermore, a new Disney world chosen for this game effectively can't appear again in present time, so going with strictly new worlds becomes a question of what yet unused films can be considered "expendable" so it's not a big deal if Sora & friends never get to visit them.
There are 2 additional possibilities. The 1st is any world featuring characters who are immortal in lore, like gods or anyone from a world that's inherently timeless in its nature. It would be far less of an issue for such characters to remain unchanged after 75 years, they're not expected to. The last option that I see is films that have a built-in past/prologue aspect with enough of a gulf in time that Xehanort could visit the film's prologue scene as a world & the film's present could still potentially appear in later KH games. Admittedly, there are very few films where this is even viable unless Disney is willing to let Square do something like depict Scar & Mufasa as cubs or whatever.
Above all of this, it's important that the worlds Xehanort visits contribute to shaping his overall worldview that even peaceful people are hiding their darknesses & are putting up a facade of being good natured. That the weak strip power from the strong & act like they earned it. That they use hollow justifications for their actions. This will be the 1st game to truly have a villain protagonist (as opposed to Terra being a manipulatable idiot) & thus is gonna have a different interpretation of the events of whatever worlds he visits. This raises yet another factor to consider of what Disney will even be ok with, they may not be keen with a main character calling their stars corrupt beings. For this thread's purposes, I'll just assume a best-case scenario on the Disney approval front.
Also, I have no idea what will constitute the enemies of the game since this is before both heartless & unversed. Maybe Nomura's making yet another new monster type that ties in with whoever the final boss even is. A Xehanort game starts to makes less & less sense once you start pondering the actual game part of it. Plus the mobile monetization factor that it'll no doubt have. But that's all for another thread.
So with all of that in mind, I got a vague resemblance of a shortlist:
1.
TOYBOX. I'll start with the big doozy of my brainstorming. In most worlds of KH3, there was some rationale for why each org member appeared in the world they did: Luxord had already appeared in Port Royal, Marluxia's flower element meshed with Corona's woods, Dark Riku foreshadowed Dark Baymax, Vanitas wanted scream energy, Ansem & Xemnas have their own histories with Twilight Town, Xigbar's in ancient Greece which is known for its equally ancient mythologies (or because he's a cyclops). YX in Toybox is not as obvious. Because he's "kid" Xehanort? Eh, still a little old for the age demographic IMO. Because he likes to play games with Eraqus? Flimsy connection. Ansem or Xion would've made better choices, one known for possession & the other a puppet, both reflecting what happened to many of the toys in the store.
So what if it is because YX has actually been there before? Woody is a very old toy, having been created in the 1950s in the Toy Story timeline. PX could have a world based around the Woody's Roundup toyline with Jessie, Bullseye, & Stinky Pete being among the other characters to appear. The plot could deal with the in-universe show's cancellation & how the toys subsequently became abandoned by the kids that once played with them. YX & Woody could have opposing outlooks on the relationship between children & toys, with YX the pessimist & Woody the optimist. It would be amazing if an earlier encounter between Woody & YX had fueled some of Woody's screw-you speech in KH3. Although Woody didn't remember much of his origins in the films either so maybe not.
Then there's the Verum Rex connection. Not that I have any idea how it would come into play in a time period before video games, but Toybox & VR are connected & we don't know how yet.
2.
Wonderland sans Alice. Alice will not be born for some time but the other Wonderland inhabitants are fair game.
3.
Neverland. How long has Peter Pan been a kid anyway?
4.
Halloween/Christmas Town. OMG I want Xehanort to meet Santa & get handed loads of coal so bad now!
5.
Hundred Acre Woods. It's literally a book.
All 4 of these are worlds filled with characters who, while not outright malevolent, are self-centered, unsympathetic, & somewhat unlikeable at times. Pan's a kidnapper who picks fights with pirates because of his hate for grown-ups, Pooh & Tigger make Rabbit's life miserable without ever recognizing it, Halloween Town's & Wonderland's populations are just plain mad. It works so well here, the characters Sora became friends with YX views with contempt for their behavior.
6.
Earth from Wall-E. Wall-E spent 700 years gradually cleaning up the planet-wide junkyard, all the while creating his own personal collection of knickknacks he grew fond of. The entire world is one massive monument to humanity failures & self-indulgences. Perhaps no other film could convince YX that humans are a lost cause stronger than Wall-E.
7.
The Star Wars Prequel Trilogy. & here's where I start losing you all. The entirety of the main 9 episodes of SW span nearly 70 years in-universe. It's almost too convenient how to fit them in the KH timeline. YX sees a world based on the prequel trilogy while a future KH game covers the Sequel Trilogy. This does mean the classic trilogy would get left off which is unfortunate.
It's become a growing viewpoint that the jedi order in the prequels was pretty misguided if not outright corrupt. Being utterly dogmatic in their beliefs, having little understanding or sympathy for the personal issues their key members are dealing with, conscripting children & indoctrinating them all doesn't make them look too good. They would majorly fuel YX's attitude that people who claim to be good are really full of it.
8.
Timeless River. This would be hilarious, YX being at TR when it was actually happening. For bonus lulz, he runs into Sora visiting form the future. Maybe then they could finally explain the damn time travel door that violates the taboos of nature or whatever.
9.
Atlantica before Ariel. Anyone remember King Triton's fear of the keyblade? Nomura sure didn't. Triton's supposedly the son of Poseidon & thus could be a very long-lived being himself. Xehanort could very well be the one who pissed him off so much & made him fear the keyblade from then on.
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