For Harvey Dent, I get the feeling that whoever is cast will be someone we would never associate with the role (My crazy idea: Taye Diggs, not because of cultural equality)
I look at all these suggestions I've heard for Dent, and they sound exactly like the suggestions for Joker. Everyone seems to be casting the comic book version of TWO-FACE, not Harvey Dent.
Actors like Liev Schreiber, Lucas and even Pearce appear interesting because we can all see the look of Dent and his eventual dark side. The problem is we no longer care about Dent, we want to see Two Face.
Now if you have an actor like Taye Diggs (who you would never see as Two-Face), all you'd be seeing is Harvey Dent. That makes it easier for you to care about the character, so that his downfall is all the more tragic.