While Superman isn't around to actually start the Justice League, his effect on Batman directly led to it, and that is something that will be heavily explored in the new film.
Superman actor Henry Cavill spoke about how his character changed Batman during a press appearance in China, one of the seldom few appearances he's made with the rest of the cast.
"Superman to the League is a, he has a sense of what is missing," Cavill said. "He has completely changed Batman's perspective of these characters with special abilities and powers."
Cavill went on to say that he essentially restored Bruce's faith in people, and that causes Batman to seek out others like them. It's been some time since Bruce mentored anyone, and fans will see him struggle with that returning role in Justice League.
“This is a very different dynamic for Bruce Wayne from the first movie, where he was full of anger and resentment and a kind of irrational rage towards Superman," Affleck told SFX Magazine.
The Batman in League will be much closer to the character fans know and love from the comics, someone who's struggled with opening up to others because of past pain, and much of that coming out of the shell is
"Here, he’s really in the mode of your more classic Batman story, where he’s more heroic, trying to save and protect people, trying to build this unit," Affleck said. "That was a totally different thing. The idea that this character is trying to assemble a group and get them to work together. Because Batman is a kind of internal, dark, conflicted guy. So it was challenging for him to turn outward and try to get all these people to work together.”
Luckily Batman will have some help forming the team, as Wonder Woman has also taken Superman's example to heart and has joined the crusade. The two will act as leaders of several new heroes, including The Flash, Cyborg, and Aquaman, and at some point, hopefully, Superman will be included in that as well.
Fans can see how it all plays out when Justice League hits theaters on November 17.