This.
People keep saying Mario doesn't have anywhere to go for options for Mario Kart, but the real issue is the team tend to add easily made filler drivers to the roster like Baby Rosalina, Tanooki Mario and Pink Gold Peach and just make locales up constantly.
There's still plenty to tap from other Mario-verse franchises and it seems weird they're not doing it:
- DKC only just got Dixie in Tour, Diddy, Funky and Rambi as a kart are the only real DKC specifics. K. Rool, Cranky, the 64 Kong Clan, other animal buddies as karts or some locales from DKC in general would be great additions.
- Wario's series are both gold mines that are completely untapped. No WarioWare outfit for Wario on a bike even. Nothing. WarioWare has a colourful cast with a fair few having their own vehicles they could bring to the race(Ashley's broom, Mona's moped, Dribble and Spitz' taxi, Orbulon's UFO) and you have the likes of Captain Syrup fitting very well with the various airship tracks and karts.
- Yoshi: Despite Kamek being both Yoshi's arch nemesis, Bowser's right hand man AND planned for both Mario Kart 64 and Mario Kart 8, he still hasn't made it. Not even in Tour. What the hell? Beyond him you have the other babies from Yoshi's Island DS(Baby DK and Baby Wario) and Poochy. Track wise, the crafted and wooly aesthetics could make for some really interesting set pieces.
- Paper Mario is utterly untapped. The amount of track potential in this is insane. We got nothing.
- Mario + Luigi is again untapped. Has it's own unique cast members like Fawful and memorable locales. I'd love a track where you drive around the insides of a giant Bowser. That'd be really creative.
- Luigi's Mansion doesn't get the shortest end of the stick as it tends to get a track, King Boo and the Poltergust as a kart, but E. Gadd was planned for Mario Kart DS and removed. Now if you want a filler character we got Gooigi who is canon and well liked, we have Hellen Graveley and Polterpup and almost every level of the hotel from Luigi's Mansion 3 would be a unique track idea.
- Captain Toad (and Explorer Toadette) just made it into Mario Kart Tour, but his tiny box worlds would make for a great Baby Park style track.
- 3D Mario cast members who have basically disappeared from the extended world; Toadsworth, Plessie, Sprixie Princess, Cappy, Petey, maybe even the Broodals etc
- Other species who constantly miss out: Boom Boom, Pom Pom, Piranha Plant, Chain Chomp, Spike and Goomba have all shown they're capable of Smashing, tennis and partying, so why not bring them in too?
- Retro Mario is also basically untapped. We have one kart referencing Wrecking Crew and not much else. There's heaps of oldschool characters who could be revived: Stanley the Bugman, Foreman Spike, Wanda, etc just like Pauline...speaking of whom;
- Pauline, Nabbit and King Bob-Omb also only just made their debuts in Mario Kart Tour which is kind of crazy. DK JR only just returned in Tour as well. Get them in the next game(and keep Diddy in it please).
Don't get me wrong, I really liked the Excitebike and F-Zero levels and I'd be fine with a few more thematically appropriate guest racers(like Fox, Samus, Falcon who all at least have their own vehicles) but the name of Mario Kart is too big a brand name to throw away and at it's core it should remain MARIO Kart IMO.