Speaking as a huge Star Wars fan, I don't know if I would want to see it in KH or not. As others have noted, due to the immense lore and potential areas, it would probably be difficult to implement properly. I would still take it, though, if they could think of a good enough storyline for it. As for Marvel, although I have never been the biggest Marvel fan, I loved Into the Spider Verse, and I honestly think that would be great for KH. The storyline and characters would mesh pretty well, and the visual design would fit in KH's aesthetic as well. Spider Verse is definitely my first choice for Marvel in KH.
This confuses me a bit though. Everyone keeps saying Star Wars has this enormous lore that can't possibly fit into the Kingdom Hearts story, yet Marvel arguably has more of a vast amount of characters and stories to follow. Not to mention all the multiple universes and different continuities/timelines. The fact you pointed out Spider-Verse is proof of that. Of all things to include I think Spider-Verse would be one of the last ones to use as an introduction to Marvel in KH. We want SDG to meet Spider-Man for the first time and have it be this whimsical, awesome experience. Not a confusing, jumbled up one where SDG meet multiple versions of the same character at the same time. At least the Star Wars story is far simplistic by comparison if you just incorporate the original trilogy. Or even if you include all three SW trilogies it would still be miniscule compared to Marvel.
Coming from someone who has always been more into Marvel Comics than Star Wars, mostly Spider-Man and X-Men since the early 90s, I am in the belief Star Wars has at least a slightly higher chance of making it in a KH game before Marvel does. There is just too much to figure out with Marvel. Again I ask which versions would they settle with? The live-action MCU versions. The cartoons from the 90s? The cartoons from the 2000s? Would it just be some bizarre combination of all the above? Marvel also has MANY planets/worlds within it's universe as well. X-Men, Thor, Guardians of the Galaxy, and Silver Surfer are just a few examples of that.
Back to Star Wars, they could either start us out in the original trilogy/original story or start way at the beginning with the prequel trilogy/original story. I don't think there is that much of a demand for the sequel trilogy to be included, with all the reception those movies have been receiving lately. Darth Maul is a badass character, in my opinion the only good thing about Phantom Menace, however Darth Vader is obviously the far more popular character and quite possibly THE number one popular character of the franchise..Having Vader in KH as an adult opposed to young Anakin would make sense for a marketing standpoint alone. Maul would make a good optional boss battle yes I am all in for that too, but Vader is the one I am sure more fans around the globe would be excited to indulge in a boss fight with. SDG teaming with Luke against Vader and the Emperor seems too good a deal to pass up.
You know in hindsight maybe I should have put other things on the list too after reading about others discussing Roger Rabbit. Traveling between the 1940s noire and Toon Town would make for a unique world. Solving different cases and doing for clue searching puzzle games would give it some diversity amongst the worlds. Fighting the Weasels would be hilarious, and as for Judge Doom, that would be an epic battle. It would be hilarious to observe how Sora would react to seeing Jessica Rabbit. Horny teenager Sora incoming. Only problem with that world I think is in the KH universe Mickey and the gang already live at Disney Castle while in Who Framed Roger Rabbit, Mickey and crew live in Toon Town. Unless they create a story where Mickey, Donald, etc. had to migrate from Toon Town to Disney Castle for some reason like maybe Toon Town got corrupted by darkness and Mickey was too weak to combat it in the past or something.
One more thing, something else I completely forgot to put on the list and it maybe has a greater possibility than all the rest...The Muppets! Disney officially owns the franchise now. What would you all think about Kermit and the gang being in KH? The Muppet world could be the new "rest world" like Hundred Acre Wood. Or maybe meet the Muppets in Radiant Garden or Twilight Town.