Slow burn.
The other idea I heard this week, treat it like Fight Club. Dent and Batman are hunting down a mysterious criminal throughout the film. This criminal is offing Black Mask’s goons, interfering with Penguin’s shipments etc. The whole time, this criminal turns out to be Harvey himself (the deranged half of his personality). I’ll add my own thoughts now. So, this reveal completely breaks Harvey. How could he have been hunting himself this whole time? He snaps and the gangster killer side of himself takes over “permanently”.
Not sure when the acid is thrown into his face. I’d assume Oz or Rupert would do it in this universe. But whenever it happens, it cements his look as “Two Face” coinciding with his breakdown at the beginning of the third act.
Freaks run wild. Two Face, Penguin, Black Mask, Ventriloquist (whoever the villains are) wage an all out war on each other before turning their sights on Batman. A price for his head yadda yadda.
Batman wants to save Harvey, who reminds him of his mother who was never given the proper care and who was surrounded by people close to her who felt ashamed of her illness. Batman tries to get through to Harvey at the very end, successfully bringing out the sensitive side of Dent, realizing there’s nothing to be ashamed of. Use this window of opportunity to turn yourself in for your crimes and I guarantee you’ll get the right treatment. This works better if Bruce already befriended Harvey early in the film. Even better if Gilda is there to support him too. They’d take him to Arkham where Batman checks in on the mental state of Edward and Joker as well. Setting up Batman’s ongoing concerns with the treatment of patients in the asylum. Sounds like a pretty good set up for a Scarecrow or Hugo Strange “abusive psychiatrist” concept for the third movie too no?
This is what I’d do if you want to actually use Two-Face in the sequel (thanks to the Fight Club suggestion on reddit that I thought was interesting). Otherwise the options are to use Harvey Dent in a handful of scenes, nothing more. Or you straight up ignore the character throughout the entire trilogy. Which is nice too because you can use fresher faces.