An idea I had for a 2012 story (almost like a scrapped episode) where the turtles get turned into humans after getting hit with a mysterious chemical compound that was one of Baxter Stockman’s attempts at making retro-mutagen.

Except humans are much more fragile than turtles, as the Shredder finds out. And on top of that, Donnie, being so much bigger than his brothers, wasn’t given enough of the chemical to fully mutate and is slowly dying. So the boys have to figure out a way to turn themselves back into turtles before either the chemical or the Shredder kills them.