Is that plausible though? The Smash Bros. games are huge: huge roster, huge level selection, and essentially countless trophies/stickers/whatever. Smash 4 might not have *more* content than Brawl, but it's still expected to have a ****ton, plus rebalancing returning characters/balancing new ones. I guess what I'm getting at is either:
-A small amount of development would go towards Smash Kart, giving us a half-baked racing experience or one that's very light on content.
Or
-A large amount of development would go towards Smash Kart, and the main modes suffer.
I think replacing Subspace Emissary with karting is a nice idea, but I'm betting the reason we got SE at all was because it worked pretty much the same as the main modes of play, whereas Smash Kart would be NOTHING like the main modes. So while SE was like developing an alternate mode within Smash Bros., Smash-Kart would be like developing an entirely different game... one that would eat up development resources that could/should be going towards the "real" Smash Bros, and one that would essentially compete with Nintendo's own Mario Kart.