The Dark Knight was a perfect movie, with two minor caveats.
While the Joker recieved exceptional development and a fitting conclusion (one can simply assume that he is locked up in a maximum security prison), harvey dent/two face was not explored fully as a character.
The transition from Dent to Two Face was far too rapid. In the comic books, dent had a history of psychiatric problems that were confidential and thus unknown to anyone before he ever became two face. It made sense that he could go mad. But in the movie, it portrays a strong willed character, who just because the person he loved dies, abandons all logic and goes completely mad. That's not realistic character progression. A person doesn't simply go from sane to insane in an instant.
Two Face needs to come back in Batman 3. It makes perfect sense that Two Face was locked away at Arkham Asylam by Gordon without anyone's knowledge. If anyone found out that dent went crazy, the entire mob that was prosecuted by Dent could go free. Batman didn't check his pulse, he had no way to be sure that Dent was dead. There was no body shown at the memorial for Dent. So it makes perfect sense...