If Warner Brothers decides to go the Darkseid route with Justice League, things can still work--even if Marvel uses Thanos. You just have to do everything (acting, writing, directing, casting, scoring, etc.) better than Marvel does, and you have to give your film a different tone than the films Marvel has. Tonally, we'll probably see something moody and epic like Man of Steel or The Dark Knight Rises. This article will focus on writing and where Will Beall should draw inspiration from.
Mr. Beall has two jobs to do. 1) He has to tell an amazing Justice League origin story. 2) He has to tell an amazing Darkseid story. And at most he has 2.5-3 hours to tell his tale. His task would be made easier if he borrowed small parts from certain comics in the same way that Nolan borrowed heavily from Frank Miller's The Dark Knight Returns. Not all of the comics that I recommend are great comics, but they do have legitimate bits and pieces to borrow from.
For Mr. Beall to tell a good Darkseid story, I recommend that he borrows from:
1) Final Crisis by Grant Morrison
Premise: What happens when evil wins? That's the Justice League and every being in the DCU have to face when Darkseid and his otherworldly legion actually win the war between light and dark.
2) Legends by John Ostrander and Len Wein
Premise: The villainous Darkseid has a scheme. With his vast resources, he will cause the people of Earth (including members of the U.S. government) to despise and rebel against their heroes. Then he will be able to conquer the planet. That is unless the heroes of Earth can unite into the greatest incarnation of the Justice League ever seen.
*Note: You can probably tell that I'm a fan of turning the entire planet against the Justice League. This could be an opportunity for Darkseid to harness the psychic energies of the classic Justice League villain Starro. If you use Starro though, make him as Lovecraftian as possible!
3) Superman/Batman: Supergirl by Jeph Loeb
Premise: Batman discovers a mysterious teen-aged girl with super-human powers and a connection to Superman. When the girl comes to the attention of Darkseid, the evil overlord of Apokolips, events take a decidedly dangerous turn.
For Mr. Beall to tell a good Justice League story, I recommend that he borrows from:
1) Justice League: Origin (Only issues 1-4) by Geoff Johns
Premise: In a world where inexperienced superheroes operate under a cloud of suspicion from the public, loner vigilante Batman has stumbled upon a dark evil that threatens to destroy the earth as we know it. Now, faced with a threat far beyond anything he can handle on his own, the Dark Knight must trust Superman, Green Lantern, Wonder Woman, Cyborg, Flash, and Aquaman. Will this combination of legends be able to put aside their differences and come together to save the world? Or will they destroy each other first?
*Note: While this story falls apart in the 5th and 6th issues, it starts with a lot of genuinely great ideas and moments. The Justice League should be the ultimate team, but they shouldn't start that way. The audience wants to see how they grow into the most iconic band of superheroes ever. And that's why Justice League: Origin should be borrowed from. That and the fact that it manages to mix humor with drama to keep the story from getting too bleak. Furthermore, basing the film on the current iteration of the Justice League could bring new fans into the comic book realm.
2) Justice League: The New Frontier by Darwyn Cooke
Premise: The New Frontier is the epic tale of the founding of the Justice League. Strangers at first, these very different heroes must overcome fear and suspicion to forge an alliance against a monster so formidable, no one hero can not stop it. If they fail, our entire planet will be "cleansed" of humanity.
*Note: This story is notable because it wasn't just popcorn fun. It had something to say about the time period it was set in and humanity in general. Powerful themes like the ones found in this book could push the quality of DC's movies ahead of Marvel's flashy fluff.
Other Sources For Special Moments:
Bruce Timm's cartoon is filled with epic moments. Check some out in these videos:
And I think everyone knows the Grant Morrison's run on Justice League has a plethora of breath-takingly badass moments too.
Am I forgetting anything? I'd like to see your suggestions for source material below!