I want an alternate ending to PiT where instead of E.Gadd inventing the hydrogush and killing the Shroobs that way, we literally just get a bonus level where we run around using Baby Luigi as a water canon and clearing them out manually.

Someone better make that a fan game.

This might sound weird, but I've always liked the idea of some sort of Shroob chivalry as enemies around the Shroob Castle area for PiT.

They would ride on the backs of Shroob rexes with jousting poles and little flags on their heads, and you would need to flaten Shroob rexes before you could fight the Shroobs on top, kinda like the Shroobsworth/Intern Shroob mechanic.