You know, I wouldn't necessarily be that opposed to recasting the Joker. Ledger was fantastic, but I can think of others who could pull it off. And the Joker is usually played as a force of nature anyway, so the step to an immortal, indestructible idea is a short one.
The only problem was that Ledger's Joker didn't die onscreen. But you know what would be kind of great? If Gotham's state executed his ass. Because that's what any reasonable state would do.
(The Joker's mental defect pleas have always
been extremely problematic, after all, and completely implausible for Ledger's nihilistic terrorist.)