So I was thinking about Joker Folie A Deux the other day and came up with a theory.
I believe that there was supposed to be another movie after Folie A Deux, but after Folie A Deux bombed at the box office, plans for any more films were scrapped, which is why the movie feels "unfinished" because it wasn't supposed to be the end. Here is how I think the next part would have gone.
After Harley hears that Arthur is dead, she is devastated and realises that she actually did love him after all. Heartbroken and enraged, she tracks down prosecutor Harvey Dent whom she blames for the death of Arthur to kill him. She throws acid on him, attempting to kill him, but he doesn't die, he instead becomes Two Faced and now Gotham has yet another villian.
It is discovered that Arthur did not actually die, he somehow survived the stabbing and escaped from the morgue his "body" was sent to after it was believed he was dead. On escaping the morgue, "Arthur" is gone and now all that is left is Joker, pure evil insane Joker. His first order of business is to track down and kill the woman who broke what was left of his heart, Harley.
He tracks down Harley and attempts to kill her, but Harley confesses her undying insane love for him and swears that there is nothing she wont do for him. Joker then makes Harley prove her love for him in some insane way which she does and Joker decides to keep her around instead of killing her, however, unlike before, he is cruel and abusive towards her, creating the "Mad Love" toxic dynamic between Joker and Harley.
Years later, Bruce Wayne is now an adult, haunted by the death of his parents when he was a child, he has had enough of the state of Gotham, where evil seems to thrive and villians are popping up everywhere so he becomes Batman, and vows to clean up Gotham for good.
This is sort of the path I think a follow up movie was going to take simply because
- The first Joker film showed Bruce Wayne witnessing his parents murder
- Harvey Dent is introduced in Joker Folie A Deux
- Lee only really becomes "Harley" towards the end of the film, she is just getting started as her new persona
This would bring everything full circle, the long battle between Joker and Batman, the toxic relationship between Joker and Harley Quinn and the creation of Two Faced