In Batman v Superman: Dawn of Justice, it was established that The Joker had murdered Robin. That lines up with what we've seen in the comic books, of course, as the Clown Prince of Crime killed Jason Todd in cold blood, leading to the Dark Knight displaying his sidekick's suit in the Batcave as a constant reminder of his greatest failure.
Filmmaker Zack Snyder has hinted that the Robin killed in the DCEU was Dick Grayson, not Jason, suggesting he planned to borrow some ideas from Frank Miller's The Dark Knight Strikes Again.
Regardless, Suicide Squad put The Joker front and centre, but didn't really have him interact with Batman beyond a quick chase sequence that was more about Harley Quinn than it was the Harlequin of Hate.
During the closing moments of Zack Snyder's Justice League, we returned to the post-apocalyptic world that had been decimated by Darkseid and Superman (who had fallen under the influence of the Anti-Life Equation). However, much to all of our surprise, the Caped Crusader was working with The Joker to try and put an end to the Man of Steel's tyrannical reign over Earth.
Now, a new photo of Leto on set as this "Knightmare" Joker has been revealed by Snyder on Instagram:
"Our theory is that [the Joker is] the one who knows where the Kryptonite, what shards of Kryptonite exist," Snyder previously said of what brought Batman and The Joker together. "He has that information. And so he's being carried...he has basically made a deal with Batman. 'Don't kill me, and I'll show you. I'll get you a tool to fight Superman.'"
"And so [Batman] is kind of stuck with him, in that way. If he kills him, or gets rid of him, then he's screwed himself."
As for what the villain actually did to Robin, Snyder refused to share too many details in case he gets to tell the story down the line (he still hopes to #RestoreTheSnyderVerse), but did say, "At that point, [Batman’s] real vulnerability was Robin. What’s obvious is that the Joker understood that. Clearly, he blew him up. Or, it looks like he burned or scorched him."
Leto's time as The Joker is over, and he's now considered box office poison after starring in back-to-back flops like Morbius and TRON: Ares. As for whether Snyder could tell more DC stories, we're not banking on it.
Still, it's a shame that Snyder hasn't been allowed to finish off his Justice League trilogy in a comic book or animated movie (still, even that would likely just increase the demand for a live-action adaptation, the last thing DC Studios needs or wants).