Eats Lunch at 4:00 pm
As of currently, all Mario Kart entries are stuck with Grand Prix, Vs Mode, Time Trial, and Battle Mode, and it has been this way since Super Mario Kart. Personally, I found it on the barebones side of things, with not much stuff to do for the single player other than race on cups for all cc's over and over and over again.
The most I want to ask for is like, a simple career or story mode where you're out doing certain tasks, taking large inspiration from Sonic & All-Stars Racing and Team Sonic Racing. I'm aware we've had a Mission Mode for Mario Kart DS that is a bit similar to the concept, but I find it extremely restricted (you're forced to be stuck with a character and kart rather than pick your character and kart) and it serves as a side mode anyway that actually doesn't unlock any content outside of the mode. Instead of most things just being unlocked randomly like in Mario Kart 8 or 7, they can tie costumes or kart parts behind the mode. Oh, and the thing with this single player mode is that you can play it with multiple people, so in a multiplayer-oriented racer, that at least gives people with local friends something to do rather than wait for their friends to finish the game.
They could also do a full-fledged hub world with locales to drive to like Diddy Kong Racing or Crash Team Racing. It was always fun driving around and finding secrets and what not, but that means they have to design a good one in order to make that interesting rather than just busy work from point A to point B.
Game doesn't need a complicated story, but I think a story would be at least somewhat entertaining for people who like dumb plots like those in Mario Party. Probably can just introduce an OC that hosts the races so at the very least Bowser can be playable in the single player mode (and give excuse for characters to have extended dialogue in the game, think the Mario Baseball titles). Would also give them incentive to introduce boss races and battles, like in the non-sequitur SM64DS bosses in Mario Kart DS but at least have a reason behind their appearance. Idk, I just want to like, bash or race the various bosses from various Mario games such as Gobblegut or tossing bombs against Mechawiggler in a New Donk City race track or something.