DCEU: 9 Things We Learned From 4Chan's Latest DC Movies "Leak"
2. Jared Leto's Joker Will Appear In Batgirl
The Rumour
There have been discussions over Jared Leto's Joker appearing in Joss Whedon's Batgirl film, most likely in flashback.
Response
It seems pretty much inevitable that the Joker will appear in the DCEU again, given the end of Suicide Squad, though Leto's future is less-clear. He didn't receive the best response, and wasn't too happy with some of the decisions made on the film, but he is almost certainly contracted for multiple movies.
An appearance in Gotham City Sirens is possible, since Harley Quinn's in it, as is a role in The Batman. But the one being discussed here also makes sense, since the Joker is such a big part of Batgirl's story.
The flashback element suggests that we'll be seeing a brief adaptation of The Killing Joke, which is one of the seminal Joker stories and a defining (if divisive) moment for Barbara Gordon.
There've been reports the movie will draw from the New 52 version of the character, which features a healed Batgirl, so the flashback could be used to help explain the psychological scarring of the character, which fits with Whedon wanting to explore the damage behind the character.