Heath Ledger was looking forward to playing the Joker in another Batman movie following his performance in The Dark Knight. The actor's casting as the Clown Prince of Crime was widely criticized at the time it happened, but he ultimately went down as one of the greatest cinematic antagonists of all-time. Ledger's iconic turn broke new ground for the film industry as a whole, netting him a posthumous Best Supporting Actor Oscar and setting a new bar for movie villains. Michael B. Jordan used Ledger as a source of inspiration for Kilmonger in Black Panther, for instance.
The character survived the events of The Dark Knight, leaving the possibility for a return in a sequel. Unfortunately, that never came to pass as Ledger tragically died in January 2008. Out of respect to the late thespian, director Christopher Nolan opted to make no mention of the Joker in trilogy finale The Dark Knight Rises. Since there was no script for a third Dark Knight movie in place at the time of Ledger's death, it's impossible to know if Nolan would have used the Joker again. But Ledger was eager for another collaboration.
In an interview with news.com.au (via Batman-News) last year to promote the I Am Heath Ledger documentary, Ledger's sister Kate shared this heartbreaking story about a final conversation she had with him:
“I spoke to him the night before (he died) and we were laughing and joking. He was so proud of what he had done in Batman. And I know he had plans for another Batman. He loved working with Chris Nolan and Christian Bale and Gary Oldman. He just had the best time ever doing that film... When he came home at Christmas he couldn’t wait to tell us all about it and he was doing the voice and laughing, showing me all the rushes. We had a great time.”
Many viewers have theorized Ledger's Joker would have had a role in a followup, but nothing official has ever been said on the matter. At the time of The Dark Knight's release, Nolan wasn't sure if his rebooted series would see a third installment, so there weren't any preconceived plans like there are today in the age of the shared universe. When he came around years later to develop Rises, Nolan had little choice but to craft a story that would not require the Joker's inclusion. Recasting the role was an inconceivable possibility, as anyone would have paled in comparison to Ledger's interpretation. It's ultimately for the best Nolan didn't touch the character again. Reaction to Jared Leto's Joker in Suicide Squad (released eight years after Dark Knight) was incredibly divisive, so one can only imagine how fans would have responded to a new Joker in the Dark Knight sequel.
The Joker was mentioned in the novelization of The Dark Knight Rises, giving the character a fittingly ambiguous resolution. He was said to be the lone inmate at Arkham Asylum, but the book also left the possibility the Joker had escaped and was out there somewhere. Nobody knew for sure.