Ascended Fanon: Baby Rosalina was created by fans after the release of Wii (due to her and Waluigi being the only humans without a baby counterpart) before she was officially introduced for 8.
Cash Cow Franchise: Got even bigger with Mario Kart DS and Wii, the latter actually selling better than Grand Theft Auto IV, which would have been unthinkable the previous generation. And Mario Kart 8 managed to be the-best second selling game the weekend of it's release (selling 1.2 million units) behind Watch_Dogs. For comparison, Watch Dogs is a Multi-Platform game available to all consoles, while Mario Kart 8 is exclusive to the worst selling console of the current generation and it itself managed to increase sales of just the Wii U by 666%, with 82% of all copies being in bundles, making it very much the Wii U's first proper Killer App.
In Mario Kart 7, instead of Mercedes Rose reprising her role, Rosalina is instead voiced by Kerri Kane. No explanation is given for this change. In 8, she is voiced by Laura Faye Smith, her actress from Super Mario 3D World.
Ludwig von Koopa was previously voiced by Mike Vaughn like Iggy Koopa, but is now voiced by David J. Goldfarb in 8. The change is very noticable.
Vanilla Lake had different music in the beta version of Super Mario Kart.
Mario Kart 64 was called Super Mario Kart R during early development, and featured a Magikoopa as a playable character instead of Donkey Kong. The feather from Super Mario Kart was also planned to be in the game, but was removed for unknown reasons.
The Golden Mushroom and fake item box were supposed to be in Super Circuit. Their unfinished forms can be accessed by hacking. The game was also called Mario Kart Advance, but that name only stuck for the Japanese release.
Mario Kart DS initially had Professor E. Gadd as a playable character, with the Poltergust 4000 being one of his signature karts. Later in development, his spot on the roster was given to Dry Bones, and the Poltergust 4000 became one of Luigi's karts.
During early development, Mario Kart Wii was called Mario Kart X. However, the name was changed because Super Smash Bros. Brawl is known as Smash Bros. X in Japan.
Waluigi was meant to be playable in Mario Kart 7, but got cut due to time restraints.
Mario Kart 8 originally had drills installed to the karts and bikes, so the player could tunnel through the ground. However, this idea was scrapped, because the dev team thought anti-gravity would be more interesting.