It's still a long way off till Mario Kart 8 gets released, so this thread may come a little too early, but I'm quite excited about this game. 

The track designs look to be the most ambitious ever by far. There will be vehicle customisation(you'll see it as well if you play close attention to the trailers) and most likely the biggest character roster of any Mario Kart game. Graphically it perhaps is the best looking Wii U game so far. 

I don't think a Mario Kart game has ever been able to attain a ´90%+ score on metacritic, but this might be the first one. I think it will score 91%. 

85-90 seems safe.

The gameplay looks solid, it looks great visually. If Mario Kart 7 is any indication, then the multiplayer should be amazing as well. Its a sequel to a long-running franchise though, so it won't get extra hype a "new" IP usually would.

90-95. Something really bad would have to happen with this game in order to have a lower metacritic score to be honest.

75 and I bet most reviewers will agree that it's too much like Mario Kart 7, so that will take a lot of points off the game.

The game gives me a feeling like MK64. I think it should score 85-90 at least.

I'm not so sure. So far only 2 MK games have gotten a 90+ score on Metacritic (Mario Kart: Super Circuit & Mario Kart DS). Console Mario Kart games have never hit that high unfortunately: 64 got 83, Double Dash 87, & Wii 82. I personally would love it if it got 90+ though, Double Dash is my favorite MK currently so the idea of something even greater is exciting :D

~82/100

In what way is it better than modnation?

90% because it does new things and seems to use the power of the console.

In what sentence in the OP is Modnation even mentioned?