DLC characters would certainly be cool but having 30 to begin with is pretty good already. I'd definitely prefer DLC courses, and I'd love to see them add in some proper battle arenas via DLC.
Id say if they included 5 characters(for each weight class) and an additional cup as a pack that'd be pretty good. They could have each cup include 2 originals and 2 Retros. Have three packs spread out between summer until the holiday season where they release an ultimate version with all the DLC included for the Holidays. The characters could include:
Light:
Prof. E Gadd, Sprixie, and Dry Bones
Medium-Light:
Cranky Kong, Kamek, and Nabbit
Medium:
Diddy, Dixie and Bowser Jr.
Medium-Heavy:
Birdo, Dry Bones Bowser, and Plessie
Heavy:
King Boo, Petey Piranha and Honey Queen Bee