How about Harvey Dent and Gilda Dent as a tag-team murdering duo? Harvey is the public face of honor and justice, but when the bad guys get off on their "technicalities," Gilda comes in as the Hangman and delivers the final "justice." But the killings are specifically coordinated with Harvey. Or rather, "Big Bad Harv." So Harvey and Gilda are picking off mobsters one by one, forcing Penguin to reach out to Batman with an under-the-table proposal. Find out who's taking us out and I'll be your eyes and ears within Gotham's organized crime ring.
The movie ends with Gilda dying and Harvey being disfigured and incarcerated at Arkham. Then he's one of the major players in the third film set in Arkham that I am trying to will into existence.
With Bruce's transformation at the end of The Batman, "duality" is a perfect theme to explore in the sequel. The struggle with balancing two halves of self that are intrinsically opposed to one another.
Two-Face is quite literally the perfect choice. I'm just very wary of using him again without doing something markedly different than Nolan's take.
Harvey Dent vs Big Bad Harv. Duality.
Harvey Dent (light justice) and Gilda Dent (dark justice). Duality.