Theory: One Giant Clock Tower

Abstract
World 7-4 from Super Mario 3D Land and Tick Tock Clock from Super Mario 64 DS and Mario Kart 8 are all part of one giant clock tower, with each appearance just taking place on a different level.

Arguments

 * Argument 1: All three locations are clockwork themed areas, which involve traversing through a series of winding gears and clock-related mechanical contraptions.


 * Argument 2: In Super Mario 64 DS and Mario Kart 8, it is demonstrable from the layout that Tick Tock Clock is a clock tower, also apparent from the fact the location is accessed and manipulated by Mario jumping inside of the clock representation in Peach’s Castle. On top of this, in Mario Kart DS, the aesthetic of the course is taken 1:1 from Super Mario 64 DS, meaning that the two depictions of Tick Tock Clock were intended to be a part of the same location. In Super Mario 3D Land, the world map depicts a clock with gears for World 7-4, and the soundtrack of the level is outright called “Clock Tower,” cementing all of the locations as not just clockwork themed areas, but specifically as very similar clock towers, with the Mario Kart 8 and Super Mario 64 DS iterations being the exact same location confirmed.


 * Argument 3: In Super Mario 64 DS, we can climb all the way to the top of Tick Tock Clock, but can find no exterior clock that would reasonably give it the title of clock tower. However, in Mario Kart 8, we see the titular clock at the top of the tower, along with a glass roof that does not resemble the ceiling from Super Mario 64 DS, heavily implying that the appearance of the latter is on a lower level of the tower than the appearance in Mario Kart 8.


 * Argument 4: The specific aesthetics of both Tick Tock Clock appearances align very closely to what we see of World 7-4, ranging from small background elements to outright blatant features, such as the massive gears, yellow grating, spinning cranks, and the clockwork related movement aspects. Also, when observing the walls of both appearances of Tick Tock Clock, Super Mario 64 DS has segmented wooden squares with brighter wooden slats, while Mario Kart 8’s is all brick with metal supports. World 7-4 has the lowest point of the level depicted with segmented wood, while the rest upwards is depicted with bricks, suggesting it would fit between the lowest (64 DS) and highest (MK8) points of the tower.