UPDATE: Deadline is reporting that this initial report from THR is completely false.
UPDATE (11/21): Director Todd Phillips tells Indiewire that the reports of a sequel are false. Read the original story below.
Standalone is fine, until you make a billion dollars. Whether it was ever in the plan or not, the success of the Joker movie has apparently spawned a sequel. According to The Hollywood Reporter, dirctor Todd Phillips will return for a sequel to the top grossing comic book movie ever. Joaquin Phoenix would also reprise his role as Joker.
There was another interesting nugget of information in the report. Apparently Phillips wants to try his hand at more DC villain origin movies. Word is that he asked for the entire catalogue of DC villains, but was denied. The report does, however, suggest that he was given the green light for two of them. Let the speculation begin!
More on this as it develops.