Back to King K. Rool discussion?
I think he has decent chances, but I could also see him sharing the same fate as Krystal back in Brawl.
In Brawl, Sakurai put serious consideration into both Krystal and Wolf. Both were heavily requested characters with potential for unique movesets. In the end, he went with Wolf because he could reuse assets from Fox, whereas Krystal would have to be built from scratch.
If we're bound to get only one Donkey Kong newcomer, it'll come down to King K. Rool and Dixie Kong, and I see the latter happening. King K. Rool would have to be built from scratch, whereas Dixie Kong could be built off of Diddy Kong's assets.
Granted, their situation isn't quite the same as Wolf and Krystal, as the they were both considered near the end of the plan where there were time constraints. With Smash Ultimate, there could be enough time in the plan where we could see them both. Although it's possible, it's something I'm more skeptical about.
So for now, my prediction is that we'll be getting Dixie Kong over King K. Rool, with the latter maybe becoming an Assist Trophy as a nod to his popularity. Once again, sharing the same fate as Krystal.