Would it benefit for being NOT RELATED to Batman?
I found the flimsy connection to the "real" batman story to be a distraction more than anything. It's there just for a sequel.
This film could take place in the real world, where a crazy man thinks he's the joker from batman, or in a semi real world where batman doesn't exist and this man just comes up with this character name Joker.
All the drama and the story arch for this guy would work all the same, if not even better considering the inconstincencies with batman's world (the age difference, the Wayne family being assholes, the joker itself etc).
I feel that it could have worked even better if there was no reference to gotham, arkham nor Wayne. What do you think?