Even if characters from RPGs and such are off-limits, that still leaves you with a **** ton of characters who have yet to be playable in an MK. Here's a list of characters who are playable in non-MK Wii U, mobile, Switch, and 3DS Mario sports games, released during the height of nonsensical "mandate" conspiracy theories, who aren't in Mario Kart yet, variants ignored.
- Sprixie Princess
- Luma
- Spike
- Chain Chomp
- Blooper
- Boom Boom
- Piranha Plant
- Goomba
- Pom Pom
- Ninji
- Dolphin
- Piranha Plant
and that is just the strictest version, now add on assistants from Dr. Mario World
- Bob-Omb
- Cheep Cheep
- Huckit Crab
- Spiny
- Pokey
- Swoop
- Bullet Bill
- Peepa
- Scaredy Rat
- Sumo Bro
- Porcupuffer
- Clampy
- Crowber
- Buzzy Beetle
- Amp
- Sledge Bro
- Mechakoopa
- Thwomp
- Fuzzy
- Star Bunny
- Octoomba
- Scuttlebug
- Penguin
- Ant Trooper
- Stingy
- Urchin
- Sushi
- Fishbone
- Broozer
- Conkdor
and finally, characters from 7 and Tour not in 8D
- Honey Queen
- Monty Mole
- Hammer Bro.
- Donkey Kong Jr.
- Nabbit
- Dixie Kong
- Chargin' Chuck
- King Bob-Omb
- Poochy
- Nabbit
- Poochy
There are 53 characters here, all of which Nintendo allowed to have in the game, and that's omitting variants - not all of them could necessarilly fit into a kart, but it still shows that the Mario universe is way more expansive than you think, even sticking to strictly the most well-known games, and that's not even getting into characters who made minor NPC roles in the last 2 decades, who should surely be on the table too.