Ramen Tengoku
Meiniac
He probably wanted a character that could take us by surprise, and Piranha Plant was probably the best candidate for that.I still dunno why Sakurai didn’t atleast go with Shy Guy if he wanted a basic Mario enemy playable.
Unlike most of the other Mario enemies, Piranha Plant has never even been playable in the spin-off games, usually being regulated to an obstacle or item...
Koopa Troopa, Shy Guy, Dry Bones, and Lakitu became Mario Kart staples (and Wiggler was a thing in Mario Kart 7), and Goomba, Boo, Hammer Bro, Blooper, and Spike joined in on some of the Mario Party games. But not Piranha Plant. If you ask me, him being the generic Mario enemy getting in makes a lot of sense in terms of surprise factor.