Last week, director Todd Phillips shared a script for a Joker sequel movie, titled Joker: Folie à deux. Tonight, THR has released a bombshell report stating that Joker 2 is planned to be a musical and that Lady Gaga is in talks to apparently play Harley Quinn. No rationale has been offered for the shift in genre. In fact, the movie itself does not seem set in stone yet, as Joaquin Phoenix is reportedly still in talks to reprise his own role as the eponymous character. There are a lot of moving pieces right now.
If Joker 2 really does proceed as a musical, Lady Gaga is frankly a perfect choice to play Harley in this scenario. Lady Gaga has demonstrated her acting chops in projects like American Horror Story and House of Gucci, and her accomplishments as a musician and singer speak for themselves. Since Joker is set in a different continuity than other DC films, this would not have any effect on Margot Robbie also playing Harley Quinn in those films.
The title Joker: Folie à deux alludes to a shared mental disorder between two people, and Harley Quinn is a psychiatrist at Arkham Asylum who is essentially seduced into madness by the Joker. The title makes a lot of sense in that light. The question now is just if people want to watch a Joker 2 musical, Lady Gaga or not.