Mark Hamill as the Joker for the Dark Knight Sequel
No, that is not a breaking news item. It is a call to action. Here is how I see it, without Heath Ledger, there is only one person who can play the Joker well enough. Mark Hamill has done the voice of the Joker for over a decade, and once you hear him, you know he is The Joker. He did the voice in the animated series and stole the show in the recent hit video game Batman: Arkham Asylum.
His long time co-star in the voice booth, Kevin Conroy, was asked once about how Mark Hamill can do that voice. He said that Hamill goes to another place, that he becomes the Joker. As far as live action acting, don't forget that Hamill rocked Broadway in the lead role of Mozart in Amadeus in the 1980s, and was deliciously evil in Jay and Silent Bob Strike Back. And, it's been 30 years since he was Luke Skywalker: with some green hair and that voice, no one will think Jedi.
Here's how the Dark Knight sequel with Mark Hamill would work:
Batman is forced underground, as hinted at the end of Dark Knight. Like in Dark Knight, the original Joker has spawned copycats, and Batman can't keep up with them all with the Gotham PD after him. The Gotham PD detective chasing him is named Edward Nigma. He is incredibly smart, a clean cut boy scout, loves puzzles and discovering who Batman's identity is the greatest puzzle he has ever had.
Nigma infiltrates a Joker gang to see how Batman operates. The new Joker discovers that he is a cop and laughs at Nigma's quixotic quest. "Who cares who Batsy was?" The Joker says. "You'll take all the fun out of it. Just. Like. A. Cop." The Joker shoots Nigma in the knee and leaves him behind to slow down Batman during a chase. Nigma has to leave the PD and all he is left with a limp, a pension and a burning anger to unmask the Dark Knight.
He becomes the Riddler to draw Batman away from Joker by committing outsize crimes that strike closer and closer to Bruce Wayne as the Riddler pieces together the Dark Knight's identity. The Riddler is playing with Batman, trying to see what resources and skills Batman has.
As Wayne Manor is rebuilt, Bruce has to decide if his life as Bruce is expendable. The Riddler is getting closer, and if Bruce Wayne is exposed, Batman will cease to exist. "Batman was meant to strike fear into criminals. Now it seems only to inspire them, to challenge them. Maybe a regular guy like Jim Gordon is the best answer to crime. Show the criminals a decent cop, with a family, who's not perfect instead of a rich boy parading around as a bat," Bruce says to Alfred in one dark moment.
The Joker just wants to torture Batman and Gotham for giggles; he doesn't care who Batman is. Except that he realizes that the Riddler is on to something: the greatest torture of Batsy would be to reveal his identity to the city. He captures Batman, peeks under the mask, laughs maniacally, and sets him free. Now that he knows the secret, and Riddler doesn't, he's going to torture Riddler as well. And Batman can't come after him or the secret will be get out. Joker has free reign and the only way Batman can keep the secret is to kill him.
Joker teases the Riddler mercilessly, until Riddler falls for a trap that Batman has laid and finds Joker and proof that the Joker is Batman. Riddler kills Joker, which the Joker finds hilarious, knowing the true secret will die with him. Batman captures Riddler and sends him off to Arkham.
This is just a rough sketch, it's missing an overarching theme (other than being a masked hero eventually backfires), and I have no idea who would play Riddler, but Chris Nolan is welcome to have the idea.