If they continue on the MK route, things will get stale. It'll be fun, but stale. I agree with your idea and I have thought about this for some time. Just think about the Kirby stages, Metroid Stages, Star Fox, it would be amazing. And then, there wouldn't be a reason for all the lame Koopa Kids.

Well, it's just my opinion. You have to understand, I am of the NES generation so I got invovled with all of these franchises when they came out. The idea of MK, like I said, was based on characters in the Mushroom Kingdom only. It was about playing with people from that cannon and/or universe. Link does not belong to said universe so it screws it all up. 

SSB was again, originally all about taking your favorite Nintendo characters and doing battle against other Nintendo characters. Playing as Solid Snake? Yeah, that just... no. It felt wrong and playing as Cloud would feel wrong to me as well.

Honestly... Then it wouldn't be Mario Kart anymore, it'd just be another Nintendo crossover, which I feel would take away from the appeal and impact of Smash Bros. Can't have that.

Besides their are SOOOOO many other Mario characters they haven't even used yet.
Like Peter Piranha, the Lumas, Professor E. Gadd, Mouser, how about some RPG characters like Starlow, Prince Dreambert, or Geno, if we can't have him in Smash Bros, than at the very least Mario Kart would be really cool. Or how about the Piantas from Sunshine?
-Magikoopa
Since DK and Yoshi are in it, and even though they have their own franchises, at their core, they're still technically in the same universe, so how about more characters from them. Like Diddy, Dixie, Cranky, or even Funky Kong. Or maybe some WarioWare characters like Ashley or Jimmy T. since the Wario franchise is really a spinoff of Mario.
Speaking of Mario spinoffs, let's bring in the ghosts of Luigi's Mansion as playable characters.

There is literally an infinite population of characters within the Mario Universe that they can use as playable racers in Mario Kart before resorting to other franchises that are from different universes, like Metroid or Pokemon.
-Having Animal Crossing characters and Link in MK8 is really stepping on the line as far as keeping it "Mario Kart" is concerned. They might as well rename it Nintendo Kart.
Donkey Kong, Yoshi, and Wario are at least in the same universe and can even be considered spinoffs of Mario....Very, very successful spinoffs, but spinoffs nonetheless.

Really we don't need more variants of Mario and Peach, more characters from another Nintendo IPs would be great. This gen we had for instance levels from Zelda, Animal Crossing and F-Zero, next time we could have Pikmin cup for instance or something similar.
Keep Mario Kart name (beacuse its very strong IP) but in fact make Nintendo Kart, I think Nintendo will do something similar because Zelda, Animal Crossing and F-Zero characters and track are very good received in Mario Kart 8.

Basically, just imagine Pikmin cup, Splatoon cup, Animal Crossing cup, Zelda cup, Yoshi cup, DK cup, Kirby cup, Pokemon cup, Star Fox cup..and of course few Mario cups. We already saw some of that in MK8, with Zelda, F-Zero and Animal Crossing but we needs more or that. That would be basically Nintendo Kart and I think that would be really great.