You might have believed that Joker was going to be a completely separate concern to any Batman universe and it would have been for pretty good reason. After all, Todd Phillips said it would be and the movie has been packaged mostly as an Elseworlds story set apart from the DCEU.
But what if it's not separate? What if the film's huge initial success (at least in financial terms) convinces Warner Bros to work as hard as they can to get Joaquin Phoenix to come back for another performance?
The stage is set for further development of Arthur's story and how he evolves into the Joker that we recognise from the comics and even more importantly to see how Gotham falls further from the end of the movie to when Batman rises. And even more intriguingly, Joker DOES include Batman's origin - or at least a slight reimagining of it - as the climax shows the murder of the Waynes.
In an intriguing twist, rather than Thomas and Martha Wayne dying as innocents at the hands of a criminal, they're presented as victims of a class war that Thomas Wayne himself helps kick off. He calls the poor "clowns" and inspires wrath against the wealthy classes and is killed for being a different kind of Gotham monster.
How that will play into Bruce Wayne's development and inform how he becomes Batman is an interesting question: after all, Batman is traditionally the protector of the people and it's the people, here, who react to corruption and greed by protesting and ultimately killing Wayne. Would that mean he turns against the people? Or just continue to stand against crime broadly?
In Joker, the titular character is created by Gotham itself, rather than by Batman, as he traditionally has been, which inverts the usual dynamic and Batman being created as a response to Joker is the completion of that inversion. But more than that, Batman being created by Gotham's fall changes his story in a clever way, because it suggests that Batman is fighting for Gotham not to be the worst version of itself.
In essence, this change to the story hands Gotham to Joker as its talisman - he's not a protector but a rabble-rouser - and Batman's job will be to wrestle back control from Joker and the other "freaks and monsters" who rise up in his wake. Determined to save Gotham and inspired by the idyllic view of the city that his father had taught him - though is was inspired by misguided corporate spirit in a way - Batman could now try to be the protector the city deserves, but also to make Gotham the city it deserves to be.
So while we were trained to expect this not to be a Batman set-up, it would work perfectly as the backdrop to the next Batman movie and it will be interesting to see if Warner Bros go this sort of way. They'd get a huge reaction from their intended audience if they did, that's for certain.