This would be the most boring run ever but I think a 100% or 96 track category could be a consideration. Much like the MK64 category, you go through all the tracks with 50cc 100cc and 150cc. I estimate an average run to be about 5 hours, but I think people could take on the challenge

Or what about 100% as in unlocking all the characters and vehicles?

100% means unlock all characters, all vehicles and beat twice the staff ghosts in time trial mode. I think all the GP mode (50cc, 100cc, 150cc and mirror) would be beaten with 3 stars but it seems difficult.

For 100%, I think it's best to only require at least a 1 star ranking because the 2 and 3 star rankings are only for show and don't go toward anything. For Time Trials, you'd only beat the regular staff ghosts since beating the expert staff ghosts does not do anything except just to be for show. I've attempted this once, but realizing the length, I quit the run around an Hour in. This is how I would define 100% and I'd really like to know what a mod thinks: Unlock all characters and vehicles, complete all Time Trials and beat the regular staff ghosts, beat all the Grand Pix cups from 50cc to Mirror Mode with at least a 1 star ranking, whatever else I think I might be forgetting that is important and unlocks stuff. For the timing, I think it should start on the OK screen for whatever you're about to start or start when you select Grand Prix/Time Trial mode and end on unlocking the last character/vehicle. The reason I did not include getting three star rankings, doing battle mode and beating the expert staff ghosts is because this game does not have a percentage count, so it's best to just do everything necessary and not additional things for show.

I agree with Frikkinfriks idea, but I think timing should start when pressing the empty license.

Oh yes! I forgot about the empty license. That actually sounds a lot better than what I first said. Sorry, I have not played MKWii in months, so I don't remember everything about it as strange as that sounds and it's because my original Wii broke and I've been waiting for a chance to swap the hard-drives. Seriously, I am not casually re-100%ing a file all over again just to play the game.

Maybe unlocking everything could be its own separate category. But what would we do about Rosalina? Since there's more than one way to unlock her, and one of the ways she is literally handed to you for free

We can call the category 99/100% for that matter

Rosalina wouldn't be faster or slower if you have or don't have Galaxy since you can get her by doing other things in the game and you'd already be doing those things making 99% and 100% being separate categories unnecessary.