We all knew that Two Face was going to end up in the Dark Knight. That's a given. In fact, as a long time Batman fan, I have already predicted that the Joker is going to create Two Face in this one. He may directly cause it or maybe it will follow the typical origin of Two Face being accidentally created while Batman tries to save the day. Instead of teaming up against Joker, Two Face will act in his own way to beat the Joker through more "violent" means.
Based on the trailer, Harvey Dent and Bruce Wayne will start off as rivals as well (for Rachel Dawe's affections) which will be significantly different. I'm open to the idea that Two Face won't be evil, just misguided in this route to take down the Joker. Although Two Face has always been a villain, he has his owns strange sense of justice. With a flip of a coin, he decides the fate of the world.
This sequel will be compared to Iron Man, but they really should not be compared. Iron Man is much more light-hearted and truly a comic book on the screen. The Dark Knight is less of a comic book, but more of a director's take on the Batman mythos. This looks like a great summer.
Also, I just realized that I may have spoiled the film. Well, in the words of Stephen Colbert, "tough titties..." Or was that Rob Cordry?