I still remember EGM’s Mushroom Kingdom Hearts rumor back in 2007.
You’ve already seen Nintendo and Squaresoft and later Square Enix collaborate on multiple projects.
– Super Mario RPG: Legend of the Seven Stars
– Mario Hoops 3-on-3
– Itadaki Street DS
– Mario Sports Mix
– Fortune Street
– DLC for Super Smash Bros. 4
You shouldn’t however expect Nintendo to be included in any way with Kingdom Hearts. If Square Enix wanted to collaborate with Nintendo on another project it’d be its own separate thing, not associated with Kingdom Hearts. Lucasfilm has a better chance of being involved with KH than Nintendo. Even Marvel itself already has been included in KH3 via the Big Hero 6 world.
Plus the idea of Square Enix “substituting Disney with Nintendo for a spinoff title” isn’t viable in anyway. Not only does Disney own the copyright to the series it also owns all of the original content such as the original characters and worlds to the series. You simply can’t take any Disney content out of any title, even if it is for a spinoff especially since that’s one of the series biggest draws and most of the content in KH is Disney owned. At that point you might as well make a new separate project between Nintendo & Square Enix anyway.
On a side note the only time Nintendo has ever been involved with a Disney licensed game was Mickey’s Speedway USA.