After 2 years I've taken my Wii out of the closet last week to play with friends, and it reminded me how good MarioKart Wii is. So I searched to see if something had been done, and good surprise, for almost 2 years, you can play custom maps (there is something like 128 maps in the pack I dl !), just using a normal Wii and a SD card : http://www.mariokartwii.com/f72/play-custom-tracks-only-sd-card-no-61493.html
Didn't tested all of them, a lot are not very good, but some are fun.
A good wiki for all that stuff : http://wiki.tockdom.de/index.php?title=Main_Page
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