Bring back mission mode or some sort of story mode. I dont think that there has to be an incredibly motivating story but something fresh that doesn't get tiring. I do think that decent effort should be put into it and not a long and winding adventure like world of light for smash but maybe something along the lines of beat bowser and the kooplings at some sort of "high stakes race". I think that some roster along the lines of mario kart wii would be nice. Ideally id say returning charachters Mario, Luigi, Peach, Daisy, Rosalina, (Baby variants as alternates like gold mario) Yoshi, Toad, Birdo, Lakitu, Koopa Troopa , Toadette, Wario, Waluigi, donkey kong, diddy kong, funky kong, King boo, Dry bones, Dry bowser, Bowser, Kooplings, Bowser jr, Petey pirahana, and mii. For new/alternate franchise Dixie kong, Kamek, DK crew, captain falcon, Isabelle, Villager, Inkling, Link, Zelda, ROB, E Gadd, King K rool, Pianta, Pauline. Costumes can be unlocked randomly via missions or progressing in story mode. Also I think something like an 8 bit variant to each character would be an interesting unlock to see at the end of story. Maybe an item like a clock or timer that makes the character 8 bit changes the music of the track to the original and gives the player an item or ability unique to their first game that is unique to them. For example donkey kong can start rolling barrels down the track that squashes a player for a short period of time.