While there are currently no future films in relation to Snyderverse, the end of Zack Snyder's Justice League left a lot on the table in terms of potential future sequels and storylines for several DC characters. In particular, the dark-toned Knightmare sequence at the end showcased Jared Leto at his sinister best as Joker, standing toe-to-toe with Ben Affleck's no-nonsense Batman. Their darkly comical and sinister confrontation left a lasting impression, making spectators wonder about their previous history, which should be explored in a prequel and/or sequel.
After the horror that was Joss Whedon's theatrical version of Justice League, Zack Snyder, fortunately, listened to his fan base and finished what he started by releasing his definitive version of the Justice League. Not only was Snyder's vision more explicit in its depiction of major characters, including Cyborg and Steppenwolf, but also there were major appearances from characters in smaller roles, including the Joker, Lex Luthor, and Deathstroke. The most striking addition was the confrontation between Batman and the Joker.
Whedon's Batman in Justice League made the Dark Knight way too vulnerable and weak in comparison to the other heroes. He was made out as a costumed hero with no powers, and was very divisive towards the league. By contrast, Snyder's cut brought back Affleck's Batman in a much stronger light, making him a leader who was very adamant on bringing the heroes together. He was also more involved in the terrifically-staged action sequences.
While Batman and the league are successful in defeating Steppenwolf, the Dark Knight continues to have Knightmares, signifying that there are still dangerous threats to humanity like Darkseid, as well as potentially horrific outcomes in the future. The Knightmare sequence at the end of Snyder's film is especially haunting in depicting an evil Superman who seeks retribution for the death of Lois Lane. Aquaman and Wonder Woman are non-existent, and alongside Batman are Cyborg, Flash, Deathstroke, Mera (an Atlantean), and Leto's Joker, who is much darker here, both in appearance and in tone, compared to his screen time in Suicide Squad.
While still darkly comical, Leto's Joker in Snyder's film truly looks like an insane inmate from Arkum Asylum, with the SWAT vest and prison clothes on him, along with the dark red lips and teeth, white makeup with facial scars, and green hair. When he speaks directly to Batman about how he is a flawed hero who knows all too well the meaning of loss and suffering, the Dark Knight feels that pain and anger within himself. As Joker stares at Batman, giving him an eerie laugh, Batman has a response of his own, even feeding his nemesis with the f-word, declaring that he will absolutely kill him in a slow and satisfyingly brutal manner. Along with that, the Joker disgustingly takes out a Joker card from his mouth, offering Bruce a truce that while they will assist each other in this Knightmare, their rivalry will continue to the death.
This bleak confrontation between Batman and Joker is reminiscent of previous confrontations with the two iconic characters, including Christian Bale and Heath Ledger in The Dark Knight, and Michael Keaton and Jack Nicholson in Tim Burton's Batman. Joker is always the laughing villain who looks like the weakest link but is sneaky when it comes to hatching up secret plans, while Batman is the tough guy trying to shut down the clown without breaking his own code. What makes Affleck and Leto stand out is that, while Batman has rules, this dark knight won't hesitate when it comes to killing bad guys, and Joker has an army and weapons at his disposal if war is imminent. There is also enigma surrounding their history, leading spectators to question what Leto's Joker did to Affleck's caped crusader to tick him off.
Back in the dark-toned Batman v Superman, Bruce has a costume at his home that appears to belong to Robin, and it says, "Joke's on you Batman," which could mean that Joker killed Batman's partner in crime. Due to this and other heinous acts, Batman has wanted retribution towards the clowned prince of Gotham.
Tim Burton's Batman was about the Dark Knight taking down Joker to avenge his parents' death. Christopher Nolan's The Dark Knight involved Batman proving to the Joker that Gotham was filled with good people and that everyone is not bad, while the Joker successfully showed the city that people in power like Harvey "Two-Face" Dent can easily fall due to corruption and misdeeds.
Affleck's Batman and Leto's Joker could've had a superhero picture that was grittier, gloomier, and more violent than its predecessors by making Batman's Knightmare come true, and a Joker who didn't have a care in the world about any human being, and killed people with his twisted mind games. There also could've been a prequel that displayed the beginning of their rivalry, and the death of Robin, along with his partnership with Batman (without the rubber-nippled costumes).
If there were to be a sequel, it could've potentially included not only Batman's rivalry with Joker, but also his battles against Harley Quinn, Deathstroke, and other key nemeses. Of course with the Snyderverse most likely over, and Robert Pattinson taking the role of Batman, this is all what could have been. However, with the success of Zack Snyder's cut of Justice League, anything is possible.