The first trailer for Zack Snyder's Justice League paints a bleak picture, opening with Darkseid's intimidating debut, Superman's brutal death and then going straight into the smouldering wreckage of the fallen Hall of Justice. This particular scene, of course, is from Snyder's Knightmare sequence, which was a glimpse of the future where Darkseid won and the forces of Apokolips overran Earth. However, in this expanded version of Earth's future, there's a devious Joker Easter egg that links to Batman's role as an aggressive hunter in this warped timeline.
In the Knightmare sequence, Bruce hunted Parademons and other soldiers who submitted to Darkseid and the Anti-Life Equation. At 0:15 in the trailer, the Omega symbol is burned into the ground, but amid the debris and warships scorching the terrain, a Joker card floats on the air. It's the Clown Prince of Crime's signature card.
Toys for Justice League as well as 2016's Batman v Superman: Dawn of Justice made clear the Dark Knight actually had this card taped to the butt of his gun during this period of being a freedom fighter and resistance leader. The technical and/or functional purpose of the card isn't known, but it's likely just a simple Easter egg, though potentially one that plays into when Joker spray painted the dead Robin's suit in the DC Extended Universe. That said, Batman could be carrying the card as a motivator to stop the next threat, in this case the corrupted Superman.
As for the card's origin, it's not clear if the one shown in the trailer is the same one Bruce keeps as a memento or if it's just a piece of debris. There's also a theory Superman could have killed the Joker Injustice-style to stem crime the way Batman never could, so it's possible Bruce is keeping this as a reminder of how far the Kryptonian went.
Ultimately, it's nice to see Joker getting some airtime, because he wasn't in the Snyder/Joss Whedon version of Justice League that released in 2017. This quick cameo, though, keeps the evil, sadistic clown alive in some way, even in the darkest of timelines.
Justice League stars Ben Affleck as Batman, Gal Gadot as Wonder Woman, Henry Cavill as Superman, Amy Adams as Lois Lane, Jason Momoa as Aquaman, Ezra Miller as The Flash, Ray Fisher as Cyborg, Jeremy Irons as Alfred Pennyworth, Diane Lane as Martha Kent, Ciarán Hinds as Steppenwolf, Jesse Eisenberg as Lex Luthor and J.K. Simmons as Commissioner Gordon. Zack Snyder's Justice League will premiere exclusively on HBO Max in 2021.