Zack Snyder's Justice League trailer has DC Comics fans wondering if the version of Jared Leto's Joker that appears in the film was inspired by the Batman: White Knight comics series. The full trailer for Zack Snyder's Justice League revealed the promised scene between Jared Leto's Joker and Ben Affleck's Batman, set in the "Knightmare" timeline that we first saw in Batman v Superman: Dawn of Justice. The trailer shows Leto's Joker sporting a cop's kevlar vest and sitting on a cop car. That's an image of The Joker that DC fans have seen before, in Sean Murphy's milestone series Batman: White Knight!
If you're not familiar with the series, Batman: White Knight is one of DC Comics' alternate universe realities, in which the history of Gotham City plays out very differently. Joker is cured of his psychosis and has his "Jack Napier" persona once again come to the forefront. Jack Napier is just as shrewd as Joker, without the burden of being a sociopath; instead of crime Jack allies himself with Dr. Harleen Quinzel and goes into politics. Jack's focus is convincing Gotham City that Batman is the greatest source of its ills, acting as a beacon for crime, madness, and corruption. Napier gets the people on his side by allying with the impoverished Backport section of Gotham, and its leaders (Duke Thomas).
With his Joker cunning, Napier uses Mad Hatter's tech to control Gotham's villains and orchestrate chaotic fights with Batman, while Napier uncovers deeper scandals in Gotham's powerful elite, like real estate schemes using areas damaged in Batman's battles, or Bruce Wayne's secret taxpayer slush fund to cover Batman's damages. With that information, he's able to blackmail and frame Batman as the villain in the people's eyes, thereby consolidating public and political power as a councilman who funds a whole new anti-crime unit: the Gotham Terrorism Oppression unit.
The imagery of Joker/Jack Napier wearing the GTO uniform comes from Batman: White Knight #8. That finale chapter saw Napier and Batman finally team-up, in order to take down the "Neo Joker" and save Gotham. Zack Snyder definitely (if inadvertently) seems to give a wink to that part of DC franchise history, with his image of Knightmare Joker. And why not? Sean Murphy's Batman: White Knight has been so popular that it inspired an entire "Murphyverse" wing of the DC Universe. That includes a sequel series (Curse of the White Knight) and a sequel/spinoff (Batman: White Knight - Harley Quinn), which have also earned a lot of fan acclaim.
Zack Snyder's Justice League streams on HBO Max starting March 18th.