The Bell Cup comes with the Animal Crossing pack. 'Bells' are the currency in Animal Crossing. Ironically, the Animal Crossing track isn't in the Bell Cup proper, but in the separate Crossing Cup.
The Sneeker, being a giant shoe that you ride in, is a reference to the Kuribo's Shoe.
Five of the Staff Ghosts make reference to the worlds in which the Koopalings were fought in Super Mario Bros. 3.
Morton is the Staff Ghost for Bone-Dry Dunes, the Desert Land.
Wendy is the Staff Ghost for Dolphin Shoals, the Water Land.
Iggy is the Staff Ghost for DS Tick-Tock Clock, a giant clock.
Lemmy is the Staff Ghost for GCN Sherbet Land, the Ice Land.
Ludwig is the Staff Ghost for 3DS Piranha Plant Slide, a Pipe Maze.
Why is GCN Baby Park entirely anti-gravity? Consider that you get a speed boost from bumping in such a section, it leads to only one conclusion: The course is Bumper Cars, an amusement park attraction!
Also, why is the course now slanted? It's because in Double Dash!!, Baby Park shared its music with the battle stage Tilt-a-Kart!
The new Rainbow Road threw many for a loop. Think of it this way, though. N64 Rainbow Road has a mystical, enchanted, magical aesthetic to it, as does SNES Rainbow Road to a lesser extent. What's the logical way to differentiate a new Rainbow Road from that? Make it a constructed, man-made, spectator track.
Baby Rosalina's introduction. Confusion arises because Rosalina's implied to be canonically thousands of years old in the past games. However, who says that the time travel method that were used to bring the other baby characters to the present can't go that far out into the past, and that Baby Rosalina can't be taught to drive a go-kart/bike despite both probably not appearing at that place in time?
The problem with Baby Rosalina isn't Rosalina's age, or Baby Rosalina being able to drive. The problem is that her design contradicts Rosalina's design in her storybook, which had notably darker hair.
Metal Mario's animations are different from the real Mario's by how stiff and sort of robotic they are. In contrast, Pink Gold Peach has animations that are much more fluid than the real Peach's.
At the start of the second and third laps of Mute City, and the second and third segments of Big Blue, the racers get a speed boost at the finish line not there at the start of the race. If you were familiar with the F-Zero franchise, you'd remember that the second lap is when boosts are given. So basically, on the second lap, YOU GOT BOOST POWER!!!
Even more brilliant, when you finish Mute City and Big Blue, you are rewarded with the race results music from the original SNES version of F-Zero! However, that song only plays after you have completed all the courses in a cup. Well, placing Mute City as the last race in the Egg Cup, and Big Blue as the last race in the Bell Cup, makes the song incredibly fitting.
Why is Luigi suddenly looking so mad most of the time in this game? It's because there were no new Luigi tracks in Mario Kart 7 and absolutely no new or retro Luigi tracks in this game! (Not even in the DLC!) To make things worse, Mario gets 3 tracks (counting Mario Kart Stadium), Wario gets 3, Shy Guy, the Dolphins, and even the Thwomps get their own track! No wonder the guy looks so upset. That being said, GCN Luigi's Mansion comes back as a retro battle map in Mario Kart 8 Deluxe.
Why is Link so good at driving a kart? Ganondorf was able to use a computer in Super Smash Bros. Brawl, so it's possible that Link would know how to drive.
One of the rejected concept art Links for The Legend of Zelda: Breath of the Wild is a Link with a motorbike.◊ Given that the BoTW Link by around the time of his DLC was solidified into the one we got, it's possible that this Link is Biker Link repurposed. Heck maybe the concept art was what caused them to have Link be DLC in the first place. Link then eventually got a Bike DLC in Breath of the Wild, albit with a totally diffrent desgin aside from the "Motorcycle horse" theme. That said it's called the Master Cycle Zero, indicating it's likely some kind of prototype for the Master Cycle that appears in Mario Kart 8. THEN Even later, the Master Cycle Zero is added to Mario Kart 8 Deluxe along with Botw Link as the new standard Link costume (his old look being relegated to an alternate). This has gone full circle.
Why doesn't GCN Luigi's Mansion use its old music (which it shared with the GCN version of Bowser's Castle), instead using a remix of Luigi's Mansion from Mario Kart DS? Because Twisted Mansion's music is already a remix of that song, and they don't want to reuse music!
Though that doesn't explain Sweet Sweet Kingdom and Dragon Palace use remixed versions of Sweet Sweet Canyon and Dragon Driftway respectively.
Whenever Koopa Troopa is a playable character. The Koopa still has a chance to get a shell, think about it for a second...
Granted, by the time the first Mario Kart game came out, it had already been established in Super Mario World that, unlike real turtles, Koopa shells are detachable and their owners can survive without them. Though that still doesn't answer where all those empty shells come from...
Paper Mario 64 had a Koopa saying something about how he had his shell custom-made and shipped from somewhere else, so maybe Koopa shells aren't even part of their anatomy. Or they are, but these are fake.
When Mario stomps on a Koopa, it loses its shell and then Mario stomps the Koopa, leaving only the empty shell. After all these years of adventures of stomping Goombas and Koopas, I think they may have found a solution to all of the empty Koopa shells lying around. That's right. Let's throw around dead Koopas' shells as weapons!
You can throw/set Bob-ombs in Sunshine Airport. That's right. You can use explosives in an airport.
There is a bit of horror for Rosalina. After the races, Baby Rosalina gets to go home to her mother and if you know her backstory...
So you're racing, and after taking a break and going to the fridge for a soda/water/what-have-you, you pause and realize that there were babies in the race. Babies whom the other characters (even Mario!) are just as willing to hurl bullets, shells, and even bombs at as they are the adult racers.
It goes both ways: These babies are just as willing as the other racers.
Inklings from Splatoon appear to be playable in Mario Kart 8 Deluxe. That makes it even worse to their own species to racetracks with underwater sections, Inklings dissolve when contacted with water (although Inklings also appear in Super Mario Maker and can swim underwater, that's just Mario in a costume).
Hearing the F-Zero announcer yell "Yeah! The final lap!" on Big Blue? Pretty cool. The fact that he says it on one of the few courses that doesn't have laps and doesn't return to its starting position? Kinda illogical. It'd make more sense in Mute City, as that one is full loop course.
Some of the anti-gravity sections can give you pause in regards to 'just why was this section of track designed as an anti-gravity section when it clear doesn't need to be'? Some are justified (Hyrule Circuit has it on the staircase, and hitting all three switches triggers the Master Sword shortcut), but then there's cases like the ski jumping hill in Mount Wario.
In the retro cups, obviously the difficulty changes because of the different in-game mechanics. However, the placing of some of the retro tracks raises some eyebrows since they don't align with their difficulty in their original game. This is best noticeable with the three Mario Kart 7 tracks in the original Retro cups: DK Jungle, which was the first course in the Special Cup in 7, is the last track in the Banana Cup; Music Park, which was the third track in the Flower Cup in 7, is now the third track in the Leaf Cup; and Piranha Plant Slide, which was the opening track in the Star Cup, is now the second course in the Lightning Cup.
In every Deluxe Battle Mode save for Renegade Roundup (meaning Shine Thief, Coin Runners, Bob-omb Blast, and Balloon Battle), the point leader (or Shine holder) wears a crown on their head. This gets a bit complicated when you're playing as a character who already has a crown, like Rosalina or King Boo, because it means they're wearing one crown on top of another.