3. Introducing all the Characters
Setting aside the issue of what would constitute an ideal lineup for the Justice League team in the film*, you have to wonder how the heck one goes about introducing all of them to the audience. It's not just a matter of "this person shows up," either. It involves establishing their situation, their powers, their personalities, their motivations,
and then making each somehow become involved in the main conflict. This is, of course, assuming that the movie will start with the heroes separate and then have them form the Justice League in the course of the story. Which seems likely, since Hollywood is so enamored of origin stories right now. Establishing one main character takes enough time. Doing six or seven or even eight is a considerably taller order. The easiest way to bypass this would be to have the movie simply start with the League already formed, and the supes already a part of it. Moving all the stuff involving getting them together out of the way leaves a lot more room to breathe for the elements that truly matter. With the team already in place, you can naturally develop each member's character as they face a threat. And this is totally doable. One big advantage that DC naturally has over Marvel is that so many more of the superheroes that make up their major team are not just known to the public, but wildly popular on their own. If you give the audience some trust, they'll be able to roll with Superman, Batman, Wonder Woman and the rest all having a history together that we aren't yet privy to. This seems like such a no-brainer to me that the only reason I can see that they wouldn't do it this way is that, again, Hollywood is obsessed with origin stories. God have mercy on poor Will Beall, in that case. Not only would he have to introduce us to these superheroes and their world, he'd have to deal with all the supporting characters and whatever threat they'd face as well. Such a headache.
*For the record, the roster of my Justice League movie would be Superman, Batman, Wonder Woman, Flash, John Stewart Green Lantern, Martian Manhunter, Zatanna, and Doctor Light. Yes, Doctor Light (The female Japanese one, just to be clear. Not the supervillain rapist (Ugh, they made him a rapist. Comics, man)). If you are somehow intrigued, email me and I'll elaborate.