But, using the Avengers as an example, how might a Justice League film actually work? Providing they had a decent budget, good production team, writers and actors, as well as unlimited access to DC's library of characters?
Well I decided to write up a rough pitch as to how I'd make a Justice League film, using the Avengers as an example and numerous pieces of DC history as background. It's kind of a stream of consciousness, but I hope that people find it interesting...
If I were to make a JL movie, I'd take a leaf from the Marvel Cinematic Universe and establish that they're all within one Universe first, via the characters own individual movies. Maybe starting with the new Superman move, Man of Steel, then going on to the possible Flash movie, the Batman reboot etc.
As for something big enough to draw them all together... well the use of a certain big bad in the next Avengers movie probably means that the New Gods have to be removed from the table in that respect. I know that they came first, but the average film goer might not know that, and besides DC seem to be running them into the ground lately. Plus it being the first film that combines their greatest heroes, it should be a chance to highlight each of the characters rogues galleries, as opposed to just having one largely unrelated one.
The plot could possibly begin with a series of seemingly unrelated mysteries, that seem ominous to the viewer as they should be able to put the pieces together before the heroes do. For example, Batman is notified of something going on when Arkham is suddenly emptied of all inmates that the Legion would consider to be useful, with some of the most dangerous criminals in the US suddenly vanish from their cells. Literally. So the Joker, the Mad Hatter, Scarecrow, Poison Ivy etc. are teleported from their cells using Zeta beam tech that Luthor had stolen from STAR Labs, and the same deal with the supermax Iron Heights prison inmates that are affiliated with the Flash's Rogues Gallery, as well as dozens of supervillain-specialised prisons and mental institutions across the world.
Then maybe Diana is brought into the situation when she's contacted by some of her old JSA teammates, who suspect that something is up with the Supervillain Underworld. Possibly a robbery at Dr Fate's tower in Salem, possibly notes passed on to Diana via an investigation undertaken by Wesley Dodds, aka the Sandman (and the first of the 20th Century costumed heroes) or something.
One of the Green Lanterns, it'd probably be Hal Jordan though I'd prefer to be John Stewart, can be brought in when he discovers that all the usual alien traffic around Earth has suddenly stopped. There are no drunken alien teenagers abducting hicks in the countryside, there are no disguised tourists, no intergalactic conmen claiming to have created the pyramids in order to sucker gullible, rich New Age people... all of them have either stopped coming to the planet, or are starting to vanish one by one.
J'Onn J'onzz has more a more direct idea of why the Earth-bound aliens are vanishing or stopping coming though, as he and many others have been abducted by the Legion. He manages to escape, and starts looking for someone that might be able to help him stop the abductions. He ends up tracking Superman down first, as he's the most public of the current generation of superheroes as well as the only publicly outed alien at this point.
Meanwhile in Metropolis, Superman is concerned by the fact that Lex Luthor has started to sponsor numerous superhero teams around the globe, and is working as Clark Kent to try and uncover what the deal is... This gives the chance to air out some members of the DC superhero catelogue, just as having a united organisation of most of the DC supervillains can show off their villain cast.
The actual scheme? The Leading Council of the Legion has decided to make examples of the superhero community, so they're both pooling their own resources and those confiscated from aliens that the Legion has robbed, abducted and dissected to learn more about their tech, so that they can launch a simultaneous strikes around the world directed at the groups in order to quash anyone from attempting to follow their examples.
Lex's new teams make up a bulk of the targets, as his company has records of their powers, weaknesses and next of kin as part of his sponsorship agreements. However, he's also taken into account that not all of the teams would die in the initial assault, so he'd be able to squeeze more money from them in some form after that they'd established they'd actual street cred.
Eventually the Martian Manhunter manages to locate WW, Batman etc. via his telepathy, and unites them together by revealing that the others have another piece of the puzzle they've been wondering about. The heroes figure out the plot, just before it's too late, and they coordinate Lex's superhero teams and each other to successfully defeat the bad guys.
The film ends with the good guys victorious, and with the Legion either in jail or scattered and disorganised once again. With Luthor's part in the scheme revealed, GL wonders what's going to become of the new, surviving heroes. Diana brings up how she used to be a member of the JSA, and the seeds of the Justice League are formed.