Dazzling Desert

Dazzling Desert is an unused track in Mario Kart DS that would have been unlocked as an extra track in the game by linking the game with Wow! Waluigi via the StarPass. It is an entire desert made up of diamond dust, giving the entire course a white, shiny hue. The track itself is very similar to Desert Hills, likely due to them sharing a desert theme. Dazzling Desert was scrapped alongside the StarPass and Wow! Waluigi, but it's data remains inside of the game completely untouched and playable. When inserted into the game, it appears with a fifth cup titled "Extra Cup" with a question mark as it's icon, likely meant to also house other StarPass courses. However, the only track in the Extra Cup is Dazzling Desert.

Description
Dazzling Desert is a white desert made up of diamond dust, likely referencing the Dazzling Diamonds. The track is roughly oval-shaped, with several mounds that act as obstacles. There are also Pokeys and an Angry Sun, the same obstacles as Desert Hills. The course itself seems to be mildly unfinished, as going off the main track makes the player fall through the floor before being respawned similarly to water, and no power-up boxes can be found. It's possible that you were meant to fall through the ground as it's made of diamond dust, but the animation may not have been finished. The track's song has not been identified as no music plays in the level once loaded, but it's likely either Desert Hills' song, or potentially even Wario Stadium/Waluigi Pinball's due to the association with Waluigi.

In the background, one can see a temple extremely similar to that of Sacred Sanctuary, the final level of Wow! Waluigi excluding Dazzling Disaster. This is likely to further connect the level to Wow! Waluigi. Some that have examined the model say there are other background areas reminiscent of Wow! Waluigi levels, such as several floating roads likely meant to resemble Rainy Roads, and a large volcano likely referencing Obsidian Ocean's second half, a lava section taking place around a large volcano.

Theorizing
As stated before, the track was planned to be connected to Wow! Waluigi via the cancelled StarPass application. Code indicating this has been found associated with Dazzling Desert and the Extra Cup. Notably, Dazzling Desert appears to have also been planned to appear in Wow! Waluigi when the link was made, as remnants of an unfinished desert level have been found, and even a fully functional Pokey enemy that appears in W!W's untitled Super Mario 64 DS-themed level. Wow! Waluigi's variant likely has less progress due to there being several other StarPass rewards the developers were working on, while Dazzling Desert appears to be the only StarPass reward for Mario Kart DS, giving them less time before the game's cancellation alongside StarPass as a whole.