It's not as easy as that. You see, Nomura has stated, when it comes to Marvel and Lucasfilm, he has to contact those subsidiaries separately in regards to their properties inclusion in the series.
Of course he has to contact them. I didn't say he didn't.
Pixar can work in Kingdom Hearts III, because, as part of the Disney Animation Studios re-structure, John Lasseter and Ed Catmull had presidency over both companies equally. And as far as live Disney movies, because that's part of the general Disney catalog and not restricted to subsidiaries out of the primary Disney pool, it's just not a realistically viable inclusion.
"Eurogamer also asked about Disney, and if they have set any limitations at this early stage yet, to which the answer is "no". There are many new and old IPs from Disney that they are currently considering to incorporate and work with, but so far, they have had free reign completely."
Eurogamer Interviews Tai Yasue - Kingdom Hearts 3, Nomura's departure from FFXV, Unreal Engine - Interviews - Kingdom Hearts Insider
When KH began, Disney was a large pool to pool from, but now with the additions of the Star Wars and Marvel properties, it is goddamn huge. Has this made it difficult to parse between which franchises you would like to insert into the ever-expanding universe?
The fact that we have to pick, and the difficulty associated with that, is something we have faced with each installment since the beginning. We pick more IPs than necessary and consult the schedule and feasibility of actually getting our designs to work. It’s been a repeating process.
Tetsuya Nomura talks Drive Forms and more with Destructoid! - News - Kingdom Hearts Insider