Let's just assume Laktiu, Baby Luigi, Baby Rosalina, Luigi, Waluigi & King Boo all get added next tour and that crossover characters (except DK characters) either aren't happening or are being saved for much later on
We still have Tanooki Mario, Cat Peach, Pink Gold Peach (everyone's favorite!) from 8. Honey Queen has her emblem in the game already so they're probably already working on her. Then for vets we still have Paratroopa, Birdo, Petey, Funky Kong, & Wiggler. And maybe DK JR for a deep pull?
Nabbit has an emblem so he'll definitely be in at some point. And after that sky's the limit on mario characters. Off the top of my head:
-E. Gadd
-Boom Boom
-Pom Pom
-Kamek
-Madame Broodal
-The other 4 broodals
-King Bob-omb
-Mary O (let me dream)
-Viruses
All of the Odyssey NPCs would also make great Normal fodder, as would Luma and all of the enemies featured in recent HD mario spinoffs (Spike, Monty Mole, Goomba, Hammer Bro, Blooper, Piranha Plant, Chain Chomp). If we dig a little deeper we can also pull on ill Piantissimo, Toadsworth (remember toadsworth? weird how he was just dropped out of the series), Lubba, the assorted NPCs from those games. Could extend to more pulls on the DK universe too, Dixie Kong would at least be a no brainer even if they didn't want to grab K Rool. Other popular enemies, like charging chuck, also seem plausible to me.
And there's a ton more costumes to grab too. Odyssey has an endless suplly and they can just make anything uo tp suit a tour.
The big ideal is they start dipping into the Wario subseries. Wario Ware characters, Captain Syrup.....r-rudy i guess....just really lean into fanservice because lord knows that's the most interesting aspect of this game to me. I'd say the RPG subseries too but...........not...holding my breath.
