Starlight Skyrise

Starlight Skyrise is the name given to a course found in few personalized copies. The course is decorated with several star shapes in both the geometry and the skybox, similarly to World Star from New Super Mario Bros. 2. It appears to be a scrapped Cap Switch level. The level is accessed exclusively by using StarPass with New Super Mario Bros. and unconfirmed reports also claim that it can appear as a secret level via personalization.

Description
Starlight Skyrise is described as a blue and cyan world with several star shapes and planes scattered around it as decoration or platforms. The level geometry is almost entirely translucent, and usually changes colour. There are usually several star-shaped platforms used to navigate to higher parts of the level, however other sky-themed objects may also spawn. Notably, no enemies have consistently been reported in this level. The level's main challenge is it's tricky platforming and use of wind to blow the player off-course.

Starlight Skyrise interestingly houses a Yellow Cap Switch, however data mining shows that it's intended colour was removed from the game, causing it to default to yellow. What colour the Switch was originally intended to be as well as its purpose is currently unknown, however some theories have connected it to New Super Mario Bros. and Waluigi (see Theorizing).

Due to its status as a cap Switch course, it's reported to be quite linear. Depending on the level of Personalization, there may be branching alternate pathways commonly including an extra Power Star or a similar collectible. Few reports have even mentioned this level being related to the unlock of Waluigi, however there has been no confirmed evidence so this should be taken with a grain of salt.

StarPass Functionality
Unfinished code left within the files seems to demonstrate that completing this level would allow the player to StarPass with New Super Mario Bros. again to receive an unspecified, unique unlock in the game. Several theories about the nature of this unlock have been speculated, however any related code appears to have been scrubbed from New Super Mario Bros. before release, so it's likely that this will remain a mystery.

Some reports, however, have claimed that a reward IS in fact granted to the player, believed to be through personalization, though these have always been simplistic rewards such as additional lives, Star Coins, etc. Leading theorists to believe that the AI may be able to briefly 'interact' with other games before it initially 'infects' the game when StarPass is used. Due to the lack of supporting code to back up this claim, no hard conclusions on the nature of this interaction can be drawn.

Waluigi Theory
This level is one of the many levels associated with rumors about Waluigi. The theory goes that the level originally housed a Purple Cap Switch that would unlock Waluigi, or in some other way be tied to his unlock. This is considered dubious, as the course's relation to StarPass would make locking a playable character behind it a strange choice however this could also be part of why the system was scrapped. The removed unlock would be Waluigi being unlocked in both games, presumably.

Starman Theory
Due to this level's theme of Stars, the common theory surrounding this level's existence is that it would be a re-implementation of the Rainbow Cap Switch, which would unlock the Starman ability. This is the generally accepted theory; however, this should not be taken as confirmation. If this theory is correct, it was likely scrapped due to problems with the fact that only Mario can wear caps. What this would unlock in New Super Mario Bros. is debated.

Testing Theory
This theory states that this level would have been used by Nintendo as part of StarPass's release to demonstrate its functionality. This is due to the fact that this level is unlocked by StarPassing 2 first-party Mario games, one of which being released very early into the DS's lifespan. The theme of Stars used in the level is also used as supporting evidence, as this would coincide with the theme of StarPass. This also explains why they would've coded it to have a special unlock in both games.