Amazingly, @mondomega.bsky.social has found 8 additional potential scrapped drivers in a file that relates to the character weight stats: Twomp, Blooper, Ninji, Goomba "twins" (presumably Goomba Tower), Chain Chomp, Fuzzy, "Bird" and "Banana".
Spiny was seemingly at one point meant to be playable in Mario Kart World. @benial.benialnet.net discovered Spiny's internal name is listed as a driver in several files. I know that there's a lot of MKWorld datamine memes going around but this is real.