Your hero is only as good as his villain, and if you're going to assemble an entire team of heroes, you better have one (or several) formidable foes to fight. The fact of the matter is, someone like The Joker or Lex Luthor isn't big enough to warrant a Justice League formation, so there will have to be some kind of global threat to get the heroes to band together. Darkseid would be a strong possibility, and is a character fans have been dying to see for years. He's often considered a villain of the entire JLA rather than being part of one character's rogues gallery, and brought the team together in the New 52 Justice League relaunch (and subsequent animated feature adaptation, Justice League: War). There are, of course, countless other villains that the filmmakers could choose, and they could even have multiple foes if need be. Whether or not Darkseid is too similar to Marvel's Thanos is also worth considering. Plus, with X-Men: Apocalypse on the way, how many giant rock-looking villains do you think the general public could withstand?
James is a 24 year old writer and filmmaker living in Portland, OR. He attended college for graphic design and writes for various sources on the web about film, television, and entertainment.
You can view all of his work on his website, www.thereeljames.wordpress.com