5 Directions Joker 2 Could Take
2. Set Inside Arkham
When in doubt, pick up where the last movie left off. Joker's chilling ending within Arkham sets up a logical sequel in the facility. Arkham has a mythic status in the Batman lore. It's spawned an acclaimed video game series, a deeply disturbing standalone comic, and has played a part in multiple movies already.
What we're yet to see is an entire film set inside its walls. Grant Morrison's Arkham Asylum: A Serious House on Serious Earth is a comic told mostly through visuals, with little dialogue accompanying the panels. It's an outright horror story that gets inside the warped mind of Batman. Every villain appears as a distorted representation of themselves. This sort of intensity is screaming out for a big screen outing. You don't need Batman to tell that story, either. The crux of Arkham Asylum's existence is that it houses the most dangerous people alive.
There are multiple paths the story can take. It could be as cut an dry as Arthur attempting an escape. In this respect, it would be akin to a classic prison movie. Alternatively, he could thrive in the hospital and gather followers like he did in the outside world. He's a figure of infamy, after all. The hospital becomes The Joker's home, his DNA running through its foundations. This is where the classical take on the character can be implemented. Arthur Fleck goes inside Arkham as a delusional psychopath in facepaint, he comes out a criminally insane maestro.
Many of The Joker's key traits can be developed inside Arkham. His psychological manipulation of doctors, his odd affection for the place, perhaps even the creation of Harley Quinn if you want to go that route. The character is eternally tied to the asylum (or state hospital in the movie), not exploring his role inside would be a missed opportunity.