Cosmic77
Smash Hero
I think Petey Piranha being a SSE boss might have helped separate Piranha Plant from Goomba, Boo, and other generic Mario enemies. For all I know, Petey may have originally been planned to return in World of Light as a boss before Sakurai said, "...Hold on a sec, I think I'm on to something here."I think we over think the "criteria" surrounding newcomer picks and that they get chosen more on a case-by-case basis.
I doubt Sakurai was like "Piranha Plant is Mario's most iconic enemy, he absolutely needs to he in Smash". He was probably more like "Haha, wouldn't Piranha Plant make for a funny and creative fighter? He's iconic enough, I'll bet we could make it work". But I suppose I'm making my own assumptions here.
But that's just another crazy fan theory. I'd laugh if it ends up being the case though.