Okay, first and foremost, yes, we have all heard the news of the alleged 2015 release date for the Justice League film, and many fans are excited, however many other ones (as is the natural trend in comic book fans) are cynical. Some say that Warner Bros are rushing it, that the lack of Tie in films will make the movie fail, that people won't know who's who, well, I respectively disagree. If I was in charge of making the movie, here's what I would do.
1. Tie in movies are NOT necessary!
Now don't get me wrong, I'd love to see the likes of Flash, Wonder Woman and Aquaman get their own much deserved movies, but I believe that with the success of Marvel's The Avengers, many people think that ALL superhero team movies now need tie in ''prequels'' in order to be a success, this is not the case. The thing about the Justice League is, THEY AREN'T THE AVENGERS, and what worked for the Avengers, may not need to be done for the JL, many people already know at least, a bit about the main cast. The average movie goer, knows Superman, Batman and (perhaps to a lesser degree) Wonder Woman's origins; they don't necessarily need to see a tie in movie to know who they are. Now Flash and Green Lantern, while not AS big as the Trinity, still have some fame. GL, thanks to the movie (no matter how bad it was) has at least some commercial acknowledgement, and the Flash's backstory can easily be hinted at in the film.
First, the plot of the Justice League film. At first, I was thinking an alien invasion, but yeah, been there, done that, so how about an alien infiltration? The White Martians could have been on Earth for decades, replacing important people with their own spies in order to observe the Earth's defenses for a /potential/ Invasion, Batman, having caught one (the mayor of Gotham) could take it back to the Batcave for analysis, where he is confronted by Martian Manhunter who explains what they are,that he has had a run in with them before and what their plans may be. It is at this time when the Martians make their move, having replaced many world leaders with their own, they can start wars among nations, civil wars inside countries,(causing both Themyscira and Atlantis to reveal their existence) and plan on weakening the Earth's defenses by starting nuclear war. They also make another discovery, that ''metahumans'' exist, with Superman already well known worldwide, and rumors of a Scarlet Speedster in Central City, the White Martians seek to study them, sending drones after them. Having defeated their opponents in individual combat, Batman, along with J'onn could find the heroes, WW, Superman, Green Lantern and a very reluctant Aquaman, and bring them together to prevent the White Martians from winning. After the fight is won, the heroes go their seperate ways, however, Martian Manhunter meets with Batman and asks what will they do if the world is threatened again, thus, giving Batman the idea of creating a "Justice League"
Run time:2hrs 45mins
Now, what /I/ would do, is the opposite of what Marvel Studios did. Instead of making the tie in movies first, make them after the JL film. We already have Man of Steel, coming out next year, and it could (although given Nolan's opinion on continuity, probably won't) be the first of the JL related films. We can then make the Justice League film, and make WW and Flash afterwards, both characters can make their "debut to the world" in the film, the world would now know they exist, and their own films can deal with the consequences of that. For instance, Now that Central City knows Flash is their, his movie can introduce the Rogues and their unease about the new masked hero. The same can be said for Wonder Woman, it could focus on her adjusting to living in Man's world after living Themyscira to work with the League; It can also focus on Ares as the main antagonist, and having Diana realize he is the main reason that "Man's world" is in so much war and turmoil.
An Aquaman film could also be sort of like this. With the World now knowing of Atlantis' existence (again, which can be shown in JL) It could deal with the government's need to learn more about this nation that lurks below, and introduce Black Manta, him being hired by the government to bring Atlantis down after their refusal to meet at the U.N . A Martian Manhunter film could take place in the past, showing Jon Jones as a detective in the 1950's, trying to solve a slue of disappearances that are caused by the White Martians who are abducting humans for experimentation. It could end with him stopping them, for now, but wondering what the world would do, if they ever returned. This could be a call back to the JL film that has already happened, and give us some nice continuity.
Now, as for a cast, that is tough, but I think I've come up with a decent one.
Superman: Henry Cavill, As stated, Man of Steel could be the first tie in film, and he could fit the bill
Wonder Woman: I think that Gina Carano could definitely handle the physicality of Wonder Woman, as well as the stunt work required, and she has acted as well, so she wouldn't be completely inexperienced.
Flash(Barry Allen): Ryan McPartlin. You may recognize him from "Chuck" I think he fits the look of Barry, and could definitely portray Barry's Nice guy attitude.
Green Lantern (Hal Jordan): Chris Pine, as seen in Star Trek, he can easily portray the ''high flying'' Hal, the dreamer who wants to visit the stars and explore the universe
Aquaman: Ben Foster. He could play a very, harsh, distrusting of the outside world Aquaman, who reluctantly agrees to team with the League for the sake of Atlantis
Martian Manhunter: Giancarlo Esposito....He's just an awesome actor,(watch Breaking Bad for proof) and could totally nail J'onn's seriousness and character traits.
And that....more or less, is how I would do a Justice League film