I ain’t saying Disney is is possible to work with, I’m saying that maybe Disney would be difficult to work with. Say Sakurai asks Nomura about Sora. Nomura asks people at Disney about it and it travels up the corporate ladder. It reaches the bigwigs, and all they see is something that benefits Nintendo more than Disney themselves. They have no reason to accept it, so they decline. That’s a possible scenario.
All third-party inclusions benefit Nintendo more than the third-party. The third-party is paid for licensing, obviously, and gets their IP shown off on someone else's dime, but at the end of the day the game is pushing Nintendo software and hardware. The majority of the profit is for Nintendo alone. And it's not like in Kingdom Hearts where there's some of Square Enix's content, but it's otherwise mostly Disney's content being used. It's just a little bit of Metal Gear/Castlevania/Sonic/Final Fantasy content being sprinkled into what's largely a Nintendo affair. It's not like third parties are getting equal billing to Mario or Zelda.
The reason to accept is that Super Smash is now a prestigious series that companies are enthusiastic about lending their IP's to, regardless of the business benefit. Even Konami, who's been burning all kinds of bridges over the past few years, held nothing back (except the Kojima fox logo). The only company that's been reserved is Square Enix, and even then they were the ones who let Sakurai pick Cloud, rather than make demands for promotional content like, say, Noctis.
And it’s not like Nintendo and Disney are on great terms. Nintendo is working with Illumination, a rival to Disney animation, for the Mario movie. Sure, Disney might not care about something like sora, but they also have no reason to accept a deal like this.
You just completely ignored how I linked to the fact that Nintendo and Disney are collaborating on a TV show for Disney primarily meant to advertise the Nintendo Switch. Disney didn't have a reason to accept that deal, either, when they could run programming that drives impressions and sales of their own properties.
Bowser was also in Wreck-it-Ralph. Disney is working with Illumination on the Mario movie, but that doesn't mean anything regarding Disney's relationship with Nintendo. Kingdom Hearts is exclusive to PlayStation, but that doesn't seem to stop Nintendo and Disney from doing things together.
Disney also broadcast the Smash Wii U EVO tournament for a year or two, which not only serves to just promote Nintendo, but shows that Disney is aware of how big the Super Smash series is. Now imagine how excited they'd be at the prospect of Sora showing up onstage at EVO.
They’re larger than Nintendo and all this would be doing would be licensing a Disney ip out for little benefit. Like I said, I’m not a law guy so i might not be educated on the intricacies of this, but that’s just something worth keeping in mind. Ultimately none of us know what goes on in the upper echelons of Disney, so we can only spitball, and based on what I know, I think Disney would be a troublesome company.
It's not even a matter of law. It's a matter of how much these companies are willing to cooperate. And all the third-parties already involved or speculated to be involved (like Disney and Microsoft) have shown that they are very willing to cooperate in various business ventures. Have we seen any company snub their nose at an invitation to Smash (besides Sega with Sonic during Brawl's development, and then they later changed their mind)? Have we seen any company be needlessly difficult regarding any character or content in Smash? Or has every company involved so far been very cooperative with Nintendo/Sakurai and what he wants to do?
Of course, it's always possible that Disney might be a stick in the mud and make the unprecedented decision to say no. There's no certainty in any of this. But the evidence suggests that there's no reason to believe that'll happen.