This is a question many people will be asking if they get around to making a Justice League film. Depending on whether DC will choose to carry on from Man of Steel or make a separate film, Superman has always been the most powerful being in the world, if not the Universe, he is basically a God. So with all this in mind why would he need help from the likes of Batman, Wonder Woman, The Flash, Green Lantern, Martian Manhunter and Aquaman? The Avengers didn't suffer from this as each character to some extent has a part to play. This is a fair question that the writers would have to get right if they wanted to make a credible Justice League film.
2. Who would the villain be?
This is another pretty big problem that the writers face. With a collective group of superheroes with the likes of Superman and Wonder Woman, the villain that would have to display some real threat to the band of superheroes. The Avengers worked because Loki was vastly powerful (not compared to Thor), was in cohorts with aliens and had the control of a worm hole. Despite the fact the Justice League can separately cite villains such as The Joker and Lex Luthor, none really pose much of a threat to League to the extent that would create a real sense of threat. If the villain lacks any threat, it will be a pretty boring experience for the audience.
3. Would they copy Marvel's template?
One reason The Avengers did so well is because the amount of planning that Marvel did before making it. They set out a well thought out plan with three phases, each concluding with a new Avengers. It has proved beneficial for Marvel to base their films out this way, so with this in mind, would DC do the same? Warners seems to be waiting to see how Man of Steel performs at the box office until they start making a JLA film. If Warners give the green light to the project it could be a while till we see a JLA film if they take Marvel's route in giving each their own films. This would allow the non comic book reading audience to get adjust and familiar with lesser known characters such as The Flash and Aquaman. But it would also mean they would have to push forward with a Batman film so close to Nolan's trilogy, tricky business, not to mention the fact it would take a brace director to go near Batman after Nolan.
4. Would they create a whole new universe?
5. Who would direct?
Warners have said if Man of Steel sets alight to the box office then Snyder will be approached to direct, judging from the trailers for the new Superman, that seems like a good decision. But if DC wanted to expand their universe, giving the heroes their own separate films then it would be essential that they find the right directors for the job. Especially for the likes of The Flash and Aquaman, theses lesser known superheroes would need a director that would attract an audience while also being a director who could nail the tone and balance of humor and action. Much like Marvel have done with Edgar Wright directing Ant-Man. If they were to remake Batman, that is likely to be their biggest challenge. Nolan's film has evolved the character so much that it would take a masterful director to make a new Batman film that wasn't as outlandishly stupid as Joel Schumacher's films and not as dark and realistic as Nolan's, not an easy job.
Whatever DC decide to do they really have an opportunity here to make a great series of films. The characters have so much to offer both comic book fans and film goers. The likes of The Flash and Wonder Woman have long been neglected by Hollywood and the likes of Martian Manhunter and Aquaman offer such an interesting opportunity for the director and the audience alike. Judging from Man of Steel they have an excellent platform to jump off from and if they can keep up the pace then Marvel have some real opponents.