Here's an honest opinion from Matt Miller of Esquire on Todd Phillips' Joker: it's in desperate need of a hero aka Batman.
I haven't seen the film myself but if the review gives a more or less accurate depiction of the story, then there are two things: one, it has done an excellent portrayal of the birth and inner workings of a villain; and two, it has successfully produced the affect it intended on the audience.
However, there is still something to be said about a sequel. Miller talks about not having closure by the end of the film. Due to the nihilistic perspective of the story, seeing that it's from the point of view of the famous Batman villain, it doesn't show a "happy ending".
And sure, it might just be setting up an origin story for Batman. Whether or not it will happen is anyone's guess. But showing this caricature of Joker could probably help give more substance to the character and to the circumstances that led him to his present situation.
In other words, the film helps us understand where the character is coming from. On the other hand, if the film becomes a box office hit, then we might see a sequel sooner rather than later.
(Image credit: DC Films/Warner Bros. Pictures; IMDb)