I think it's obvious how the movie needs to end. This franchise's films should consistently end with important resolutions and development for its two most powerful and interesting players. Sequences that not only resolve character issues for Batman and Gordon, but speak to the larger story about what's happening to Gotham, because Gotham itself is as much a character as anyone. So THE DARK KNIGHT needs to end as follows:
-With a temporary reprieve, and The Joker having gone to Arkham.
-Harvey Dent having been scarred, and them having lost a friend to the war, and possibly even to madness by this point.
-Batman realizing that things are just going to keep getting worse ,the new kinds of madness they are going to be facing.
-Batman and Gordon making a conscious decision to keep fighting his war, despite the costs to themselves and those they care about.
-With Batman and Gordon "making up" (I hope they have Gordon forced to hunt him in this one, but we'll see)
So naturally, once again, it needs to end on the roof of Police Headquarters. That much, they absolutely nailed in BATMAN BEGINS.