If you were wondering why Disney's recent Marvel and Star Wars acquisitions didn't make it into Kingdom Hearts III, the answer may disappoint you. There was nothing specific keeping Sora from hanging out with the Rebel Alliance or Avengers, except for plain old business contracts.
Kingdom Hearts 3 Ultimania, the official companion book for the most recent Kingdom Hearts game, includes a special interview with director Tetsuya Nomura. Translated by Twitter user @lunesacree via KH13, Nomura reveals why properties from Marvel and Lucasfilm weren't included in the game, and it mostly boils down to contract complications – the same reason why Mickey Mouse was barely in the first Kingdom Hearts.
"In order to change those [properties] into a game, contracts must be made with each company separately, and there are cases where other game companies already have contracts, so although the Disney Group has indeed added those [properties], incorporating them into Kingdom Hearts isn't so simple," said Nomura.
"That's also the reason Mickey is in only one scene in Kingdom Hearts 1," he continued. "At the same time, another company was releasing a game that had to do with Mickey, so though we were denied his usage, we persisted and eventually got 'as long as you only have one scene, from far away, as a silhouette, with him waving his hand or something.' Since we had to make the best of the biggest [and only] chance we had, that's why that scene appears that way."
While Nomura's comments do mean we won't likely see any Marvel or Star Wars characters in the potential Kingdom Hearts III DLC, it does mean we might see those characters make it into future games in the series. Because, let's be real, we're going to end up with more Kingdom Hearts games down the line. Probably on the Switch, since KH spin-off games tend to go handheld.