Honestly after Mario Kart 8 Deluxe and it's DLC's I think we are reaching a point where it will basically eventually be Nintendo Kart in everything except the name, I think the brand name Mario Kart is too big for Nintendo to want to change it.
Like you mentioned, just look at all the new racers and tracks in Mario Kart 8 Deluxe. We're seeing so many more non-Mario Nintendo IP's be used in them whether it comes to track based on certain franchises or racers.
Racers - Mario, Donkey Kong, Link, Inkling, Villager, and Isabelle.
Tracks- Mario, F-Zero, Legend of Zelda, Animal Crossing, Excitebike, Splatoon, etc.
I think it will still be mostly Mario characters and tracks probably making up the majority, but I won't be surprised if characters like Link, Villager, etc become a staple in the Mario Kart games from this point forward, maybe even expanding to include more like Kirby.







