In a Screen Rant interview, James Chu--the Industrial Light and Magic animation director who worked on The Avengers--has expressed skepticism about the prospect of doing a Justice League film before taking on solo films for the heroes first.
"It was like a nice little wine tasting of Iron Man," Chu said of Marvel's approach to developing their cinematic universe. "And it was like that for all the other characters. I think it was a good balance of how they introduced the characters. In that final battle, it was a balancing act when you have all these superheroes competing for screen time. I think Joss did a great job in making sure that they all had their voice and we weren’t missing them too much."
Speaking more directly to Warner Brothers's rumored plans to introduce some of the characters (including Batman) in Justice League, Chu was not totally sold.
"I don’t think it would have worked as well," Chu told Screen Rant. "I think the success of this movie is purely built on Marvel and Kevin’s vision to get to that goal. To introduce you to this group who has pulled together. You’ve seen the movie, they’re not one happy family right at the beginning they’re all fighting with each other, they all have issues with one another. So I think it was only fitting to have them be united by a common enemy – forcing them to deal with each other. I mean The Hulk punches Thor right? It’s like 'Okay I hate you, you’re a good ally but I still hate you for what you did to me.' So I don’t think it would’ve worked because you don’t have the back story of the characters, because they do function so independently and they all have their own worlds. If I just saw a movie that would start off with all of them I’ll be like, 'So Thor is a god, but how did he get to earth and where’s he…?' So you kind of need to tell those stories to tell how they got there. It’s like, 'Okay So Captain America was created from a super soldier project in World War II, but how is he here in modern time?' You can’t tell that without explaining the origin story."
Of course, DC has the benefit of a team composed largely of characters whose backstories are not only pretty self-explanatory but who also enjoy significant public recognition and awareness. Superman, Batman and Wonder Woman can be taken as a given and while Green Lantern may require a throwaway explanation, it's unlikely you'd want to do another origin for that character again in any event.
In fact, if you were to introduce a Steve Trevor or Maxwell Lord-type character to the film, the whole question could be resolved in a couple of minutes of screen time. While contemplating putting the team together, a non-super government agent could just do a run-down of the characters, their basic powers and backstories, not entirely unlike what it's implied Phil Coulson did for Iron Man's benefit in The Avengers.
Still, all interesting observations, especially coming from someone on the inside.