The Dark Knight Rises may not mention The Joker, but the novelization does
While I respect Christopher Nolan's decision to not mention The Joker in THE DARK KNIGHT RISES, it does leave a big plot hole in the film. Specifically, if Bane was looking to create mayhem in the city, wouldn't The Joker be one of the first people he would let out? Or, if The Joker was too unpredictable, wouldn't Bane have just killed him and gotten that out of the way?
While we may never know how Nolan's film would have addressed the issue, there does seem to be an answer to what happened to The Joker. Well, sort of.
In the official novelization of THE DARK KNIGHT RISES, author Greg Cox included this passage: Arkham Asylum is left almost deserted after the introduction of the Dent Act, which sees almost all criminals housed in Blackgate Prison. "The worst of the worst were sent here, except for the Joker, who, rumor had it, was locked away as Arkham's sole remaining inmate," reads the novelisation. "Or perhaps he escaped. Nobody was really sure. Not even Selina."
To have The Joker as the sole inmate of Arkham seems like a fitting answer. I find it hard to believe he would have escaped quietly and not popped his face back into the madness that Bane set forth.
We may never know what Nolan would have done if Heath Ledger had not died tragically. But, it would have been nice to have gotten that character closure within the trilogy.