It's a promising sign when Robert Di Niro is on board for a movie such as this. For a while he's been content taking easy-money comedy roles, but he rarely ever takes serious movies without having a lot of faith in the screenplay itself.
Did a quick check on the director and was quite surprised to find that the movie was written and directed by Todd Phillips. His back catalogue of work is almost exclusively cheesy comedies :-
Road Trip
Hangover I, II, and III
Starsky & Hutch
Old School
Due Date
That is, until 2013, until he wrote and directed War Dogs. His first real non-comedic work.
I don't know what this means for the Joker movie, to be honest. He doesn't have a portfolio that would lead me to have an expectations coming into this movie.