I love this series, but to be honest, I don't care about most of the characters in Mario Kart. They're generic and functionally identical. Bringing back Paratroopa as an alternative to Koopa would have no impact on me. The babies are terrible additions and don't get me started on Pink Gold Peach and Gold Mario.
Now, it would be a bit more interesting if they added characters with a backstory like E. Gadd, Kamek (thinking about the one originally from Yoshi's Island rather than the generic Magikoopa species), Pauline, and - surely a pipe dream - characters from the RPGs. Screw the generic Paratroopa, give me Parakarry. Geno, Lady Bow, Fawful, Cackletta.