When Zack Snyder took back Justice League, he decided to ensure that every bit of his envisioned storyline should be added to the cut which has led to an epic 4-hour melee of DC superhero fun. Much of the new material comes from changing character designs via VFX work, reshooting sequences for better detail, and even brand new sequences with actors Jared Leto as Joker and Joe Manganiello as Deathstroke.
Though we haven't seen any footage of Deathstroke yet, a glimpse of Knightmare Joker in the trailer has hyped up audiences and has filled the web with countless theories about the Snyderverse. Adding Joker to the narrative means that for the first time, DCEU versions of Batman and his arch-nemesis would share a scene together, not as enemies but as allies.
Talking to Total Film about his decision to shoot new sequences with Jared Leto and bringing Joker back in DCEU, Zack Snyder said that the duo's conflict is imminent if they exist in the same world. He also stated his version of Justice League could be the last time we see Ben Affleck as Batman, and because of this it would be unfair to the viewers to miss the chance of these two exuberant characters to collide on-screen in the DCEU. Snyder has long thought of Batman and Joker's role within DCEU and how the Knightmare connection between the two, which he began to set up in Batman v Superman: Dawn of Justice.
"It was a wish of mine to bring Joker into the continuing storyline. I hinted at it in Batman v Superman - when you see Batman's gun in the post-apocalyptic world, it has a Joker playing card taped to it. This has a little bit to do with that concept."
Theories about those Knightmare sequences lean in on Joker being the center point of the connection between BvS and Zack Snyder's Justice League. Guesses are being made that the Knightmare sequence in Batman v Superman is actually takes place after the newly revealed Justice League Knightmare sequence, ruling out the possibility of it all being a dream. One theory claims Joker may die helping the team secure a Mother Box, which the remaining league members would use to send Flash back in time and warn Bruce Wayne. But Batman fails. And then, in the last attempt to kill Superman, he obtains Kryptonite (as it was in BvS Knightmare), only to be captured and then killed by Superman. The gun Batman carries in Batman v Superman is Joker's gun, which we saw him holding in the Snyder Cut trailer.
With Zack Snyder confirming that the two sequences share the same context, it's possible that the Knightmare sequences would indeed be more than just premonitions. It's possible, that Chapter 5 of Zack Snyder's Justice League, titled "Change Machine", would see Flash reach Bruce at the right moment instead of being "too early." This would connect the two Knightmare sequences, but would also wipe them from existence due to the events Flash's time travel.
Joker is specifically added to the Knightmare timeline of Snyderverse, and Zack wouldn't have done it just for the sake of fan service in some throw away dream sequence. His version of Jared Leto's Joker shall play an important role in the film and will probably be be highly impactful in the overall narrative. He's previously hinted how Joker's conversation with Batman would dig deeper into Batman's persona. Having Joker analyzing Batman with his maniac-like mentality in an apocalyptic future would be quite thrilling. And we're pretty excited to witness this part in Zack Snyder's Justice League. Are you?