Man of Steel had a military officer called Captain Farris appear as a minor supporting character. It was later confirmed her first name was Carrie, causing some fans to speculate that she might be the Green Lantern villain Star Sapphire (whose real name is Carol Ferris in the comics). Ultimately she was forgotten about after two movies, and Green Lantern never appeared in the franchise before it was rebooted.
Batman v Superman: Dawn of Justice established that Wonder Woman had disappeared for a century after World War 1, with Diana saying that she turned her back on mankind in disgust due to what she experienced. It was expected that this would be explored in the solo Wonder Woman (2017)Prequel film, only for that plot point to be completely ignored. While the movie does end with Diana being less of a Wide-Eyed Idealist, her faith in humanity is never shattered, and nothing in the film suggests that she plans on giving up her career as a superhero. This is taken even further in the sequel, Wonder Woman 1984, which reaffirms that Diana was absolutely still fighting evil during the decades between the first movie and BVS, albeit while keeping a low profile.
Suicide Squad (2016)'s post-credits scene shows Bruce Wayne confronting Amanda Waller (who it's implied knows he's really Batman), and warning her to shut down the Squad before he and his new friends do it for her. Nothing further ever came from this, as Waller's team was still operating several years later without any interference from the Justice League, and if she did know Bruce's secret, Ben Affleck's departure from the role rendered that plot point moot anyway.
The entire "Knightmare Sequence" from the previous film goes unaddressed, and Batman on meeting Barry Allen doesn't lampshade that he looks familiar from seeing the surveillance footage in the previous film or Barry's face, nor does he mention the similarities of the Parademons from his nightmares to Diana, nor is there any mention of it having influenced his decision to bring Lois Lane as Superman'sLiving Emotional Crutch as a backup to Superman's Resurrection Sickness.
Lex Luthor's connection to Steppenwolf, and his involvement in helping the Apokoliptan invasion, and how exactly he came to learn about the other metahumans and unearth their Secret Identity is left unaddressed. In the film, Batman mentions reading up "Luthor's notes" indicating research of a mysterious event in the past tied to Mother Boxes, and Steppenwolf mentions offhand that "the Kryptonian's" death plunged the world into despair and fear, and allowed him and his parademons to return to earth, but it's not made clear if this was a deliberate conspiracy between him and Luthor or not.
The final shot of Batman v Superman showed dirt moving around Superman's coffin, indicating that Superman is not really dead, and that his resurrection would follow similar to the original comic, where his corpse was mistaken for dead by Earth science, and was recovered by his robots to the Fortress of Solitude to properly convalesce. In the film, this is more or less retconned with his revival being presented as Superman apparently having died for real at Doomsday's hands, Batman and Cyborg briefly trading technobabble about Kryptonian physiology, and Bruce pitching Superman's resurrection as if it was something that was always possible but waiting for the right technology, before it coming via Mother Box, but it's not made clear what Bruce believed Superman's exact condition was for him to suggest this. It is possible that he was in between life and death.
The theatrical version of Lex Luthor's meeting with Deathstroke had him proposing a team of supervillains to challenge the newly-formed Justice League. In-between the theatrical cut and the Snyder Cut, every movie was suspiciously evasive over whether the Legion of Doom would ever appear or not. Aquaman kept both Ocean Master and Black Manta alive for the foreseeable future, but beyond that there was nothing. Then the Snyder Cut confirmed this was a last-minute change of script and Warner Bros. all but confirmed that neither version of Justice League would ever get a proper follow-up.
SHAZAM! (2019) had Mr. Mind appear in two scenes, the latter of which showed him offering an allegiance with Dr. Sivana. While it seemed like this might pave the way for the Monster Society of Evil, David F. Sandberg pushed that plot aside in favour of creating some original villains in the sequel.
As much of Zack Snyder's Justice League was originally written and shot before the drastic shakeup of the franchise that occurred after the underperformance of Batman v Superman and the outright failure of the 2017 theatrical Justice League cut, the movie contains a number of Sequel Hooks for projects that never came to fruition. Due to WB subsequently confirming that their 2021 release of Zack Snyder's Justice League was intended to be the end of Snyder's story arc rather than the start of a Justice League trilogy, as well as James Gunn and Peter Safran subsequently taking over as the heads of DC Studios in late 2022 and immediately rebooting multiple key characters, these plot threads most likely won’t be resolved:
Back when Justice League was first announced in 2014, it was originally titled Justice League: Part I, with plans for Part II to be released in 2019, and a hypothetical third installment following some time after that. As such, Zack Snyder's Justice League contains multiple direct lead-ins to the next installments: Darkseid shares a brief but ominous staredown with the heroes, Batman begins turning the ruins of Wayne Manor into a headquarters for the Justice League, both Bruce and Cyborg experience premonitions of the post-apocalyptic "Knightmare" future with an evil Superman, and the final scene has Martian Manhunter arrive with a warning about another invasion from Apokolips. All of these were meant to be explored in the two sequels, but were instead Left Hanging. The cancellation of Ava Duvernay's planned New Gods film also stymied any chance at expanding upon the rest of the Fourth World, while Intergang's connection to Apokolips was completely ignored in Black Adam (2022).
A pregnancy test is briefly glimpsed in Lois Lane's drawer, which was meant to foreshadow the major revelation that she was pregnant with Bruce Wayne's child.
Ryan Choi also receives a suspiciously prominent scene at the end of the film, which Snyder confirmed was meant to tease a solo The AtomSpin-Off that was ultimately rejected by the studio.
Lex Luthor is last seen recruiting Deathstroke to take down Batman, directly setting up Ben Affleck's solo Batman film, which was planned to feature Deathstroke as the main antagonist. Due to Affleck stepping away from the project in 2017 amidst personal issues and Creative Differences, The Batman (2022) was subsequently reconfigured by Matt Reeves into a complete Continuity Reboot with no relation to Zack Snyder's planned arc. Part of this Retool meant abandoning Deathstroke as the villain.
Related to this, while the aforementioned scene in the theatrical cut where Lex mentions forming a team to take down the Justice League was a Whedon reshoot, the idea was based on Snyder's original plan for the sequel, which would have seen the Injustice League as the main antagonists, with Luthor as the group's leader. Even though Aquaman (2018) ended with Black Manta and Ocean Master (both planned to return as part of Snyder's Injustice League) defeated but alive, additional members the Riddler and Captain Cold were originally going to be introduced in The Batman and The Flash (2023), respectively, before being recruited by Lex. However, as mentioned, The Batman was reconfigured into a completely unrelated project with a radically different version of the Riddler after Affleck's departure, while The Flash was heavily delayed and extensively rewritten into an adaptation of Flashpoint, removing Captain Cold as the villain in the process.
Black Adam (2022) ends with Adam being confronted by Henry Cavill's Superman, directly teasing a future battle between the two characters. However, due to a combination of Black Adam's underperformance at the box office and James Gunn and Peter Safran rebooting the character of Superman, this plot thread was abandoned, with Cavill later confirming on social media that he would no longer be playing Superman going forward.
Additionally, thanks to Black Adam's underperformance, any plans to expand upon the Justice Society or show Adrianna and Amon becoming Isis and Osiris are effectively stillborn.
