To be honest I really dig the new Rainbow Road. Why? I kinda made a backstory to it and I think the Wii U addition follows suit.
SNES, it was formed in the sky by lumas, N64, fellow Chain Chomp inhabitants run amongst the course to smooth it out from being flat into being amorphous, GC, before the Chain Chomps had left, as they could not sustain life on it for any longer, they created several contraptions to allow them to move around better (I.E. the star cannon), Wii, meteorites slam into it, creating some holes and change some of the contraptions, 3DS, a few planet like rocks take residence, including one which has the original Chain Chomp inhabitants, and finally Wii U, man tries to contain the mysterious track for entertainment purposes, and it is reformed into a space station stadium.