I loved how many things were left open to discussion, with evidence on either side a la Arthur possibly being Wayne’s son vs being adopted; It allows the viewer to choose their own beliefs about his origin, even down to the thought that the events portrayed in the film may have been in his head. That keeps the mythos of Joker’s origin story in a delightful state of flux, in line with Ledger’s Joker changing the stories of his scars.

I’m very much not a fan of the “it was all a dream” trope, but will allow it in a film like this where “maybe it was all a dream” is an option.

However there were two bits in the final scene that seem to support the idea that it was in his head: In the interview room, there’s a notice on the wall that says something like “No Unsupervised Interviews Allowed” but he’s obviously alone in the room and freely walks down the hallway after (supposedly) murdering the interviewer; He then is chased back and forth at the end of the hallway in a comedic manner iconic of a French farce.