Don't pick Rainbow Road...

Mario Kart would make a great official LEGO theme. The crazy characters, vehicles, and courses would all translate well into official sets. But MOCs are the next best thing, and these ones by 6kyubi6 admittedly are way cooler than any set LEGO could reasonably make:

I really like the shaping on Luigi's kart and Yoshi's bike. 

UPDATE (11/21/14): Here's the full collaboration with 74louloute and fujiia (thanks to The Brothers Brick for leading me to this):