Well, a collectaphon, obviously.
Something like SM64/BK/DK64.
And i think exploration, collectables and powerups would have worked really well with Wario
Yeah, I would have to say a Wario World type game, too! Maybe played a bit more like Mario 64. That gameplay style was the recurrent theme at the time, like a prerequisite of some sort...
I think it's understandable that some games might look like that, after all, there wasn't any 3D console games before that. Mario 64, Zelda games, F-zero and others have a really good and solid quality, but you can note the unexperience in most of those games. And I think Mario Kart 64 looks and feels unpolished.
It's pretty unbalanced, too. Lightweights are better than other characters in basically every way (best acceleration, best top speed, best off-road abilities, can triple-tap accelerate, best handling), heavyweights and middleweights are statistically pretty much the same besides weight (and Bowser can triple-tap accelerate while the others can't for some reason), etc. I mean, it's still one of my favorites in the series, but it is a tad unpolished.
It's an oddity in Mario Kart 64, where Toad, Yoshi, Peach, and Bowser can quickly tap A multiple times to accelerate to top speed faster than if you simply hold the button. It works best if you quickly tap it three times, and hold it down on the third press.