When it was first shown, Mario Kart for the Gameboy Advance looked a bit different than the final version. If you take a look a these beta images, the characters had bigger heads and smaller karts. Also it seems that the tracks had some differences in the details and in the course design, as we can see in the images and the video below.
Mattrizzle of The Cutting Room Floor discovered that there are a few unused tracks still in the final game’s data. Most of them appear to be Super Mario Kart’s battle courses, which means that they were probably at one point planned to be in the game along with the Super Mario Kart tracks. Unfortunately, the starting positions in each of the battle courses don’t exist, so the starting points default to position 0,0.
Thanks to Mario125 and Goomther for the contribution!
The unused tracks: