Loads of things:
-Bring back Double Dash, HD remaster or DD2
-Battle-mode specific stages and weapons
-Mario-Kart / Rocket League spinoff
-Custom parts
-Voice chat

Like this

https://www.youtube.com/watch?v=IKvhmK82mmc

And in VR offcourse..

Copy Diddy Kong Racing story mode and hub.
Link needs to be in the game and not DLC.

I do wonder what they're gonna about that. If they expand any further it may aswell be called Super Smash Kart.

8 player local support: split screen on the NX home console + four additional NX handhelds.

- Real customize options with sell - resell online or battles with friends to win custom parts

- Create online tournament in group to win coins/parts/characters - Kart Fest

- Story mode

They could have a decent campaign mode. Something like Diddy Kong Racing did.
Or even the Sonic racing games. They didn't do anything special but it was at least better than nothing but standard races. Also the battle mode! In general, there's very little they can do to improve on MK8 though. I think it was easily the best in the series. The only thing wrong with it is that they didn't have a Xenoblade track in the DLC. I wanted to see a track on the Bionis' Leg.

- 200cc stays
- Coins for increced speed optional / max speed at start option
- Diddy Kong is playable (Villager but no Diddy come on he had his own racing game)
- A mode simular to the relic runs in Crash Team Racing
- Create a track

64 battle mode needs to come back. Maybe have tracks like Mario Galaxy, going planet-to-planet.

Air limit in subwater zones