I would appreciate MK9 being more of a live game, similar to something like Splatoon or CTR. New maps over time, new outfits and karts to unlock. I wouldn't even mind if they introduce microtransactions into this if that prolongs the time they can keep supporting this game, as long as they do in a similar way as CTR did where you can easily keep up with clearing everything in the shop as long as you play the game semi-regularly and the microtransactions are just there as a option to speed unlocking all the cosmetic stuff up for the whales and new tracks and gameplay stuff is free for everyone. Even if it's a bit more grindy than CTR was, as long as it's nothing like the terrible gatcha stuff in the mobile MK I'm fine with extending MK9's lifespan into infinity with a bit more of a live game approach.
I think all my other wants for the next MK really come from playing CTR:NF. More personality in the character animations, big roster with some way more obscure character picks along the regular cast (give me E. Gadd, Prince Peasley, Pianta, Dimentio, Paper Mario and Cackletta), costumes for characters (that amazing UK Waluigi skin better not stay stuck on the mobile MK), kart customization, a more intricate drift system to allow for a bit more skillful gameplay among the chaos, a substantial singleplayer mode.
The idea of going into a more Smash Bros or Sonic and Sega All Stars route could also be interesting, going for a more "Nintendo Kart" approach. There's still a lot more untapped potential within the Mario series though so that wouldn't necessarily be my first choice.