We've already had Baby Park in Mario Kart DS, DK Mountain in Mario Kart Wii and Waluigi Pinball in Mario Kart 7. They would be good tracks to see return though...
What would be a cool idea would be if they would have DLC for each Mario Kart Game. Maybe take the top 4 courses from each game and put them in packs? That or put all the retro tracks in as DLC so people could pick and choose what tracks they would want. It'd be a good way to make money on Nintendo's part too.
Speaking of retro tracks: What kind of worries me about this game is that if future Mario Karts don't use the anti gravity mechanic they won't be able to use this game's courses in retro cups unless they heavily modify the them.
Speaking of retro tracks: What kind of worries me about this game is that if future Mario Karts don't use the anti gravity mechanic they won't be able to use this game's courses in retro cups unless they heavily modify the them.
To be honest, I don't think that's a problem. They'll just constantly bring back the anti gravity mechanic like how this game is bringing back the gliding and underwater mechanics.