It's gonna be Crash, I can feel it.
But I do actually have a bit of evidence to back up this claim- sometime recently Sakurai tweeted something along the lines of, "A lot of you might not know this next character, but I hope you'll be able to enjoy him anyway." The article that posted this made it sound like Sakurai was trying to avoid us getting our hopes up because it wouldn't be a popular pick, BUT I realised that his tweet was fan translated, meaning it was intended with Japan in mind. So I think that means we're getting a western character, and what better western character to finish off Ultimate than good ol Crash?? And it would line up with how much Nintendo themselves is hyping this up...
Plus, they're doing some "celebration of gaming" along with all this, so seeing the 3 90s rivals (sonic, mario and crash) all in one place would be fitting. Its a stretch of course but man am I hoping this is true, thoughts and prayers everyone