8. Where Was The Joker? - The Dark Knight Rises
Chris Nolan had rough plans to bring the Joker back for the final part of his Dark Knight trilogy, but the death of Heath Ledger made that impossible. There was no way he was going to recast, and ultimately the character isn’t mentioned in The Dark Knight Rises.
This was done out of respect for the late actor, but it does feel strange the Joker isn’t referenced despite his impact on the city. The official novelisation provides some closure for his absence, however.
It explains that after The Dent Act made it impossible for criminals to plead insanity, all of Gotham's prisoners were sent to Blackgate Prison. All except for the Joker, who became the sole inmate of Arkham. The book keeps his ultimate fate vague but hints he may have escaped. In a way this is fitting for the character; he appeared out of nowhere to cause chaos, and vanishes when his prediction of Gotham tearing itself apart comes true.