I didn’t care for the musical nature of it. That aside:

The first ‘Joker’ clearly established that the main character was Arthur Fleck. Clearly suffering from mental illness as a result of abuse growing up, and the people he murdered were abusing him in some way. To me, as a long time Batman fan, this ‘Joker’ was anything but Joker.

1. He didn’t take pleasure in chaos.
2. He wasn’t anti-batman in anyway.
3. A clear back story that lined up with his behaviors.
4. Clearly a dude pushed too far (kind of like Killing Joke, but it didn’t line up with that character’s style).

However, when he was in the ‘Joker’ role, he became clear headed and focused. So now the ‘Joker’ clearly isn’t Joker but the beginning of Joker?

In Folie A Deux, we see him continue to be abused, still having strange fantasies, a system failing around him, and noticably the ‘Joker’ character is resonating with people fed up with all sorts of bullshit. The collective desire to burn it down and restart - very common theme within the Batman comics and joker. We see Harley Quinzel introduced, and as we discover throughout the movie - this is the actual Harley Quinn Psychiatry, brilliance, obsessed with Joker to the point that when Arthur says it was just something he made up to do what he thought he needed, she quit him. The last parts of the movie tie is completely together. Ricky, who is killed by the only guard that is sometimes nice, breaks Arthur, realizing murder happens to those undeserving by those who ‘shouldnt’ be doing it.

Joker escapes after the court room explosion (with a burned Harvey Dent, that was badass). He’s rescued by enthusiasts, who he escapes from. He encounters Quinn and she says that his “fantasy was all that mattered, and it’s gone.”

When the Joker is murdered at the end by the psychopath, he starts it with a retelling of the joke Arthur told Murray. Albeit, one that was significantly better delivered. He also notably uses a knife, and is laughing the whole time, and gives himself a scarred smile. This man, (if Warner Bros could ever finish a good DC series) would likely continue to be an evolution of ‘Joker’.

This all works because:

1. Joker rarely has a back story, and famously is stated to prefer his origin to be “Multiple Choice.”
2. Several comics and media (Notably the Arkham series of video games) explore how Joker is not confined to a single person. Unlike Batman who has very specic goals, values, and traumatic origin, Joker is a shared ‘idea’ between these individuals that reject the value of civilization at all.
3. Harley Quinzel was only introduced in the 90s, but her main obsession with Joker evolved over time as he abused her, or burned things she learned to care for, but seemingly remained obsessed because of some ‘fantasy’ she provided him, UNTIL he broke that fantasy and she quit him abruptly just like in the movie.

I don’t think it was a great movie. But it actually reimagined the same Joker story in a new way that I did thoroughly enjoy. And it left it plenty open for more stories from it, just as all good DC stories do.