This is my first time doing this, so please be patient.
So The Justice League movie will happen at some point, maybe in 2018 or 2019. If WB is not going to do a movie for every League member, then they will introduce them in the movie (Superman already introduced in Man of Steel) and Batman will show up in the "Batman vs Superman" movie in 2015.
So I don't think that they need to show all the Heroes in the first movie.
The first Justice League movie needs to have this heroes: Superman, Batman, Wonder Woman, Green Lantern (Hal Jordan) and Flash (Barry Allen). They should keep Aquaman out, and introduce him in his own stand-alone movie, because he is a hard character, and if the JL movie will be successful, then the WB could make 2-3 Superheroes every year, like Marvel.
I will include Martian Manhunter in the movie, but only as a supporting role.
as far as Green Lantern, I will not use Ryan Reynolds again, I will reboot the character and put other actor. I wouldn't use John Stewart, because using him is like putting Robin instead of Batman.
The villain should be Brainiac, with his evil robot army.
I made a Fan-Made trailers few Month back, showcasing the story for the movie. (the Trailer has Aquaman too, but like I said, don't think he should be there in the first movie):
So the Cast:
Superman - Henry Cavill
Batman - Ben Affleck
Wonder Woman - Gemma Arterton/Lynn Collins/Adrianne Palicki/Amber Heard
Green Lantern - Channing Tatum/Joel Edgerton/Jensen Ackles
Flash - Chris Pine/Garret Hedlund
Martian Manhunter - Idris Elba
Brainiac - Cillian Murphy/Benedict Cumberbatch
Aquaman - Liam Hemsworth/Liam McIntyre/Alexander Skarsgard (not for the first JL movie)
So the Story is that Brainiac is like a space Terminator, he has his Big ship and big army of robots, and he goes from World to world, and destroys every advanced civilizations. One of One of these civilizations was are the Martians. J'onn J'onzz (Martian Manhunter) is the only one who survives the extinction.
few years before the events of the JL movie, J'onn, (who is now going around space trying to save others from Brainiac), finds out that Brainiac has returned to galaxy, and he is on his way to Earth. J'onn trying to warn us about the extinction, but one of Brainiac's Robots attacks J'onn's ship, and he crashes on Earth, and he is then being taken by the Government, who believe he is a danger to Earth, Especially after what happened in Metropolis with Zod.
We find out this all from Martian Manhunter who tells all of this to the League, when they all meet at some point in the movie.
The first half of the movie deals with Wonder Woman, Green Lantern and Flash, each of which has its own story arc, but not origin stories.
The Heroes find out about Brainiac in the first place, after Hal Jordan witnesses Brainiac destroy some planet, and then the goes to Earth after finding out that Brainiac's next stop is Earth.
the League is coming together to protect the Earth, there is a big climax with them fighting Brainiac's army, and then Martian Manhunter Sacrificing his life to save Earth and blow Brainiac's ship up. Yes, I just killed Martian Manhunter. I did that because the character won't work well in the movies, and he isn't that interesting, so people won't want to see him that much.
From there, the road is open to a very successful franchise.
After the JL movie they must have origin movies to Wonder Woman, Green Lantern and Flash. Give us an origin Aquaman, so we could see him in JL 2. After that they can start doing movies to other heroes.
Ideas for more DC movies:
Next Stand-alone Batman movie should be based on the Arkham Asylum game.
for Man of Steel 2 or 3 they need to put Doomsday against Superman.
JL 2 should have the Crime Syndicate of America (the evil counterparts of the Justice League), base it on the Crisis on Infinite Earths story.
For JL 3, they need to use Darkseid.