The Kingdom Hearts protagonist was apparently high on the list for the DLC roster.
Super Smash Bros. Ultimate may go down as truly the most incredible official crossover ever (notice I said official, none of that M.U.G.E.N stuff). The game not only represents pretty much all of the major Nintendo franchises, lots of third party companies are in on the thing, too, and the whole project has become something truly special. If a new rumor is to be believed, it also may have crossed over with another crossover.
In the latest episode of Kinda Funny Gamecast, Imran Khan, formerly of Game Informer, revealed something interesting he’d heard about Super Smash Bros. Ultimate. He said that he was aware that Nintendo had pursued getting the main Kingdom Hearts protagonist, Sora, on the roster. Apparently, it was something they were actively pursuing, but Disney, specifically Disney Japan, rejected the request. While this is obviously going under rumor, Khan is generally a pretty credible source.
Kingdom Hearts is already a crossover between Disney and Square Enix properties. It would definitely be interesting to see that and all its craziness crossover with another crossover. Super Smash Bros. Ultimate does have a second season of fighters coming up, so it could still very well happen.