I would love to see The Koopalings dropped in the next installment, but I would add Boom Boom and Pom Pom because thry've appeared most recently in the main games. I think Nabbit being playable in Mario Golf World Tour solidified him as "entering the fold" sort to speak, as well as Spike. I also want to see Pauline being used more. She has a 3D model, so its not like they have no way to design her. Just alter her for bikes and buggies. I think this game should have alts. Alternate characters will not appear as random AI in offline modes except for Paratroopa, DK Jr., and Gold Mario. Their counterparts will never be a part of the same race however, so like DK and DK Jr. will not appear in the same race, for example. Also, increase the amount of racers to either 15 or 16 for variety.
My ideal roster would be this
STARTER
Mario (Tanooki Mario alt.)
Luigi (Fire Luigi alt.)
Yoshi (with colors)
Peach (Cat Peach alt.)
Bowser
Donkey Kong (DK Jr. Alt.)
Wario
Toad (Captain Toad alt.)
Toadette
Waluigi
Birdo
Koopa Troopa (with colors and Paratroopa alt)
Daisy
Shy Guy (with colors)
Baby Mario
Baby Luigi
Bowser Jr.
Baby Peach
Boo
Diddy Kong
UNLOCKABLE
Boom Boom
Pom Pom
Baby Daisy
Baby Rosalina
Lakitu
Plessie
Spike
Honey Queen
Metal Mario (Gold Mario and Shadow Mario alt.)
Pink Gold Peach
Dry Bowser
Rosalina
Dixie Kong
Pauline
Nabbit
Petey Piranha
Dry Bones
E. Gadd
Kamek
Hammer Bro. (Fire/Boomerang/Ice alt.)