Honestly, I'm hoping for at least the newcomers from Tour. Not the clones, but:
Nabbit
Poochy
Chargin' Chuck
Hammer Bro
Dixie Kong
King Bob-omb
Captain Toad
Etc. Those would certainly help fill out the roster, even before we get to the spinoff characters.
And dear god we need those spinoff characters. Wario Land, WarioWare, Donkey Kong Country, Yoshi, Peach Showtime, Luigi's Mansion, Mario & Luigi and Paper Mario characters would be amazing in Mario Kart, and I'd honestly prefer them to Smash Bros style crossover characters.
I bet Nintendo could surprise us by bringing back some deep-cut characters - like Diddy Kong or WarioWare's Ashley. How sick would it be to get WarioWare-style tracks to crank the chaos up to eleven in races?
Dixie Kong, Jimmy T, and Captain Syrup would certainly be characters I WANNA see but I fully expect this game to give us. Well some couple of new characters that'll please the crowd but obviously staying within mainline Mario like "WOAH you can play as Toadworth" is my threshold for how crazy they could take it with this game if they even do that
Edit: ignoring DLCs of course, I wouldn't be surprised at all if they did a couple other Nintendo ips that are relevant atm like they did with Link and Inkling. Sure wouldn't want that though, team spinoff