Why are so many people put off by the idea of another Nintendo developed kart racer? If they would release another Mario Kart in the same style, then sure, but there is no need to limit your minds like that. The Nintendo Switch has shown us time and time again that Nintendo is stepping up their game with some of their long running IP's. Besides that, Diddy Kong Racing and Mario Kart 64 were able to co exist. DKR even had record breaking initial sales at the time.
It differentiated itself by having different types of vehicles, a hub world, boss races, coin challenges, progressive power ups, secret items, a secret world. It was an adventure. Currently, Mario Kart has none of that. It also baffles me that they never followed up on those ideas. Just imagine what sort of game we could have by now if each generation they would've improved on the DKR mechanics.
A Nintendo Racers game in this style could be amazing. In DKR there were 5 different coloured balloons that served as power-ups. Red/Offensive, Blue/Speed, Green/Crowd Control, Yellow/Defensive and Rainbow/Magnet(?). If you grabbed another balloon with the same colour, it would become stronger. Red went from 1 missile, to a homing missile, to 10 missiles. Blue's speed boost increased with each extra balloon, with the third one being ridiculous. Now imagine a Nintendo roster, where each character has their own version of each power-up.
Link/Red could cycle through different arrow types, his Blue could start with carrots and end with the best sail from Wind Waker, his Rainbow could involve the hookshot while Samus/Rainbow uses the grappling hook and Luigi/Rainbow the suction from plotergust. Fox/Red can start with a single laser that shoots for x amount of times, to a double laser that shoots longer, to a smart bomb.
They could bring in jet skis for that wave race itch, sand buggies for drifting levels, drag cars for f-zero type tracks, drones for flying levels.
Challenge runs - Drift for x amount of time, have x amount of air time, block x amount of attacks, win the race without using power-ups, etc.
A huge hub world with its own races and secrets, where each franchise could get their own themed area, culminating in a boss race. All connected in a similar fashion as World of Light. A super baddy has merged all the different universes, watching from his X-Station.
This might move away from the Mario Kart 9 branding, but as seen from the DLC in 8, they are not afraid to take Mario Kart to a new universe. They also only started using numbers for 7 and 8, I don't think they will continue with it. Mario Kart has been the same since its initial release on the snes, tuned to perfection, now it's time to evolve. I am confident we'll see another Kart Racer from Nintendo, I don't think it'll be the same formula.