What did they thematically contribute that the a-listers I mentioned wouldn't have offered in their place? What "set them apart" so to speak? Its fascinating that nothing interesting was done with Metamorpho's relation to the elements, or his every changing / adapting powers. Instead we get a generic version of his romantic subplot / origin from the comics. In the end, Metamorpho was just another monster of the week sort of character. There was nothing there to justify his appearance.
Magpie is pretty much the first time we've had a character with a genuine case of split personality within Batman's rogues. I mean, there was Harvey Dent and his "Big Bad Harv" persona in
B:TAS, but that was quickly ignored in the show and was never brought into the comics. Magpie serves as a warning to Batman to what could happen if he lost his ability to control his Bruce Wayne and Batman personas. He even relates to her what it's like to lose that battle. Not to mention Magpie has a completely different motivation than Catwoman's. She's obsessed with regaining her memories and finding out who she is, while Selina is out for profit and fun initially. Magpie only compulsively steals because of her mental illness. Catwoman is sane and has a complicated, mutual relationship with Batman. Magpie is deranged, and despite her desire for a similar dynamic with him, it doesn't happen. Batman treats her as a common criminal, and when he finds out about her past, he just flat-out pities her. Margaret and Selina are far too apart to fulfill the same role in a story.
Anarky, while not my favorite, is again not too similar to the Joker beyond wanting Batman as an archnemesis. A consistent motivation of the Joker is to bring Gotham, and specially Batman, to the brink of madness. Even the Christopher Nolan version of Joker, which this show's Anarky is often compared to, has the same goal. Anarky cares more about creating chaos in order to compete with Batman. He tries too hard to be Batman's anti-thesis, and the hero calls him out on that and shows him a lack of respect or even concern, something that he would never really do with the Joker.
As for Metamorpho, keep in mind that his incident is recent and is just getting used to his powers. It took him near the end of the episode just to speak coherently! Even then, he does display some of what he can do: he's turned his skin to metal, rock, and plastic; can change between liquid, solid, and gas;
he can change the temperature of his body, and can even make creative use of his body composition, like absorbing the electricity from Batman's tasers to redirect it as a stream of lighting from his mouth or using a chemical composition to propel himself to the top of the building. Clayface can't do any of these.
Lets not forget the fact that Metamorpho in this show is meant to be a recurring character. He has plenty of time to do new things with his powers and take part in new stories.
