Neat episode! I also see what you mean about Nega-Gosalyn. She is the opposite of Tom-boyish regular Gosalyn, hardly the stuff that would make for a arch villain. Your idea of Neg-Morgana probably could work if Negaduck wanted to find a way to split up "The Friendly Four" who now protect his former charge. Brainwashing her and molding her to become the evil counter part of the good Gosalyn might provide enough motive. Darkwing Ducka along with her dad must find a way to restore Nega-Gosalyn to her normal sweet self to prevent "The Friendly Four" from breaking up and Negaduck from turning the city back to the way it was before Darkwing messed with it. Of course you would have to explain how Negaduck managed to escape from that vortex thingy. Hmmmm…Nega-Morgana might be of help there to by being the one who magically summons him from beyond because…well…she might have a thing for him? Eh, something like that at any rate.
Thank you for sharing you Youtube of that episode, it was fun seeing all those characters again.