I feel the kraang didn't get a satisfying finale, they deserved better.
Karai's partner, we never learned much about her, I don't even remember her name (shimigami?)
Mighty Mutanimals, they were featured more than enough for guest characters so I can't complain but since it's a "wish" thread then yeah I would've loved to have seen more of them.
Hun/The Purple Dragons, what was the purpose of introducing Bruce Lee Hun if they barely did anything with him? He had potential, in fact I don't think he ever straight up fought the turtles. Same with the Purple Dragons, were they a gang or just those 4 dudes? Also remember when that one purple dragon helped Leo in the first season? That never went anywhere.
April being a high schooler, I was not a fan of her psychic powers but they could've had an interesting dynamic with April still being in HS instead of completely ignoring it besides in an episode or two.
The italian mafia, they were used enough but they had a lot of potential, wish they had been used more and had an actual plot against the turtles besides the anti-mutant one.
Russian arms guy, yeah he became Rocksteady but he appeared since Season 1 and he had a lot of potential as well to give the turtles more stories.
Too bad the writers wasted all those opportunities in horror spoofs.
