Going into Justice League, producer Charles Roven said his biggest hope was that people would walk away wanting to see a Justice League 2. While reviews and early box office numbers may question whether or not everyone wanted a first Justice League, there have still been a number of positive takeaways, as many critics and audience member alike responded well to many of the film's new characters, each of which could get their own solo movies in the future (Aquaman already has one on the way), so there's still hope for a second outing, and Justice League plants a few seeds from which it could grow.
Since Justice League introduces Steppenwolf, Mother Boxes, and some Apocoliptian lore, early assumptions were that the franchise would follow a similar path to the Marvel Cinematic Universe's eventual fight with Thanos by having the Justice League face off against DC's big baddie Darkseid, and while that appears to have been the plan at one point, Justice League tied up all the various plot threads to that part of the story, with nary a tease of any additional threat coming from Apokolypse.
Steppenwolf, Motherboxes, and Apokolips related story elements are all tied up with no loose ends to lead into a larger cosmic battle, verifying rumors during production that Zack's original quasi-cliffhanger ending had been totally retooled to be more definitive. There is a Darkseid reference in the movie, a threat is most certainly still out there, but it's not one with any kind of immediacy that will dictate the DCEU's short-term plans, leaving the plot of a potential sequel wide open for a fresh story, with Justice League ultimately just clearing the slate of the more controversial elements from Man of Steel and Batman v Superman and uniting the team.
Hall of Justice Teases a Growing League
While Justice League's main plot doesn't leave any lingering villains to combat, it does tease a setup for a larger League, with Bruce Wayne making plans to turn Wayne Manor into the Hall of Justice, including a big round table "with room for more," already suggesting plans for more characters. The only other traditional Justice League member with a movie in active development is Shazam, so he's one definite opportunity for a sequel, with the eventual Green Lantern Corps to introduce Hal Jordan and/or John Stewart.
The Nightwing movie is still in production with director Chris McKay recently announcing plans for an open casting call, and Joss Whedon's Batgirl is still on the table (although who knows if that will still happen), and both of them could hypothetically appear in a future Justice League 2. Then, of course, the sequel could also follow in the footsteps of its predecessor by using the team-up as an introduction for new characters in a similar fashion to The Flash, Cyborg, and Aquaman showing up in Justice League before getting their own solo adventures.
For all the financial and critical failings of Justice League, this is the one area where it's widely seen as a success: it introduces likable characters and sets the table for the future. The problem is, the movie itself is performing so poorly that whether or not that future will come to fruition is in doubt. Fortunately, Warner Bros. had a mostly stellar year outside of Justice League, especially with lower budget hits like Annabelle: Creation and IT, and bigger films like Wonder Woman and Dunkirk also earning a good chunk, meaning even though Justice League's box office performance is a major concern, it doesn't have to be the end of the road. With James Wan's Aquaman already getting high praise, Annabelle: Creation's David F. Sandberg leading a "Big but with superpowers" version of Shazam, and Patty Jenkins and Gal Gadot returning for Wonder Woman 2 - the only 3 DC movies in active production - the future is actually far brighter following the failure of Justice League. If Warner Bros. can string together 3 or more successful solo films, then the appetite to see those characters team up again (or for the first time in some cases) could easily return.