Ceric, I think Wart and Mouser would be great choices.
I kind of wish they'd put in at least some of the Koopalings, though that would mean too many new characters. My personal favorite was always Lemmy, mostly because the way that he was portrayed in the old SMB3 Valiant comics was kind of funny.
Lakitu was a good choice, but they still could have put in a Hammer Bros, Magikoopa (particularly Kammy), or even Tatanga.