Batman v Superman: Dawn of Justice. When Bruce Wayne made his debut in the DCEU in 2016, the Caped Crusader had a vastly different mindset about the Man of Steel. Bruce feared that if Superman ever turned on humanity, Earth would be defenseless. While Bruce and Clark reached an understanding before Superman sacrificed himself to stop Doomsday, Batman had warnings about the Kryptonian's potential turn. In Batman v Superman, Bruce had a vision of a dystopian future where Clark had become a tyrant and began ruling the planet. After the events in Man of Steel, Batman wasn't willing to take the risk of the Kryptonian conquering Earth in the future.
During Batman v Superman, Knightmare Flash affirmed Bruce's worst fears, telling him that he was right about Clark and that Lois Lane is the key. With all of that in mind, Batman tried to kill Superman to prevent this future — before the two heroes eventually ed forces. However, in Zack Snyder's Justice League, the team resurrects Superman from the dead, even though Cyborg gets a Knightmare warning that Darkseid would corrupt Clark if they went through with reviving him. At the end of the Snyder Cut, Bruce has one final vision of his Knightmare team being found by Superman, who is ready to kill them. While it is, understandably so, out of Clark's control in this future, the vision proved Batman to be right this entire time in wanting to be prepared for this possibility.
That is one of the significant differences between Zack Snyder's Justice League and Joss Whedon's theatrical cut from 2017: The Whedon edition removed all the Knightmare elements that had been set up in Batman v Superman, leaving the storyline unfinished. Even though there won't be Justice League sequels, the Knightmare arc would have allowed Bruce to explore those original fears that he had. If the Snyder Cut had never happened, the additional Knightmare scenes would not have seen the light of day.
Ultimately, Bruce was right about Superman being susceptible to being corrupted — with Darkseid using the Anti-Life Equation on Clark after Lois' death. While the sequels would have allowed the Knightmare future to happen, it would never have been a permanent situation. After The Flash successfully traveled back in time to make sure they prevent this reality from ever happening, it would lead the heroes to victory. Snyder always had plans for what Justice League 2 and Justice League 3 would have looked like, and he was carefully laying the groundwork for an epic story; one of the unfortunate aspects of Zack Snyder's Justice League not getting to continue with sequels is that Batman v Superman laid the groundwork for an arc that will never be completed.
It would have cost Knightmare Flash his life by using the cosmic treill to deliver Batman that message, but it would have paid off in the end. But for Batman, in particular, this would have been something entirely new for the character's media history. Not only would Ben Affleck's version of the DC hero have had to stand as one of Earth's last defenses, but he would have sacrificed himself after they got a do-over in the present time to take on Darkseid in Justice League 3. That is why Batman v Superman and Zack Snyder's Justice League helped kickstart that arc which would have made this trilogy a significant one.
