Mario Kart World's soundtrack is so massive and all-encompassing of the Mario series that there's even a remix to a niche Mario drawing tutorial from a DSi app.
Mario Kart World has a few things players aren't too big a fan of, the divisive intermission tracks removing three-lap tracks in Grand Prix and online modes, vehicle stats being hidden, and, of course, the $80 price. But one element the fanbase is united on is that the soundtrack absolutely rips, with a common gag in the community being it's actually an "$80 soundtrack with a free game."
While the new tracks all have pretty great tunes, Mario Kart World's Free-Roam, intermission laps, and Knockout Tours all feature tons of remixes from all over the Mario series, with the likes of a ska rendition of Super Mario Bros. 3's Athletic theme; a version of DK's classic Jungle Hijinx song that sounds ripped from The Incredibles; and tunes from spin-offs like Wario Ware, Wario Land, and Yoshi's Island. But I've heard perhaps the most niche possible musical reference in Mario Kart World.
DSi game Flipnote Studio arrived in the late 2000s and was a lovely little flipbook animation software that still has an enduring fanbase today making animations with it. One cute detail from the game was a little tune that taught players how to draw Mario himself. Flipnote was produced by Yoshiaki Koizumi who has been a producer or director on every single 3D Mario title, mind you.
This could've stayed as a fun little Easter egg in the DSi app, but for whatever reason Nintendo really likes this little tune as it's been referenced multiple times since, with Super Mario 3D Land using a remix as the Special World 8 theme, and the drawing tutorial itself returning as a hidden Easter egg in the Super Mario Bros. Game and Watch released in 2020. And now it's back in Mario Kart World with a brand new arrangement.
Obviously Nintendo reckoned you'll need a lot of music to carry a big sort-of-open world game like this, and so dug to the depths of the Super Mario canon to pick out tunes, but this is on another level of niche. In fact, it seems like Donkey Kong is the only Mario-adjacent series not to get much love on the soundtrack (outside of the aforementioned Jungle Hijinx remix, there's no DK tunes), but fans reckon a DLC expansion may remedy that injustice.
