I think these characters will be in:

Kamek
Bowser Jr. (possibly Baby Bowser instead?)
Baby Wario
Toadette
Boo
Daisy
Diddy Kong
Donkey Kong Jr.
Birdo
Paratroopa
Dry Bones
Petey Piranha

It would make sense to have Bowser Jr. returning, and I would be shocked if he didn't... Or maybe he will be replaced by Baby Bowser? In keeping with the baby 'theme' we seem to have going on here... Who knows?