Okay, so I know I've already submitted Tiara, but I was browsing and I thought of someone else I think would work well:
Rex (mock up by myself-Rex surprisingly has no 3D model)
Rex is a surprisingly underused enemy who has never had the opportunity to be playable before, meaning our little Mario Kart would be his first official playable role!
We don't exactly have many 'minion' style characters at current(not even Koopa Troopa yet) but this rare Yoshi looking enemy I feel would make a good 'skin'.
Rex could actually work pretty well for both Yoshi(obvious, very similar looking) but also
Birdo.
If successful and a Birdo skin: I'm thinking we could reskin the magic potion item that Birdo uses into the key from Super Mario World(the game Rex debuted in) in which rather than using the Magic Potion to enter Subcon, he'd use the key to access the Mario World keyhole; essentially entering into a totally different world similar to the potion's effects.
If successful and becomes a Yoshi skin: We'll work out how to 'reskin' Yoshi's special item at a later date...when we decide what Yoshi's special item actually is!