THE HEROES:
Batman/Bruce Wayne - Christian Bale (Terminator: Salvation, The Dark Knight)
I doubt that he'll reprise his role for a Justice League movie, but he IS Batman. I can't think of anyone who can actually replace him. Plus his Batvoice is awesome!
Superman/Clark Kent/Kal-El - Henry Cavill (Red Riding Hood, The Tudors)
I never even heard of this guy before he was cast as the Man of Steel. I have to say, he has a good resemblance to Superman, and we'll likely see how he does next year.
Wonder Woman/Princess Diana - Olivia Wilde (Cowboys & Aliens, TRON: Legacy)
I was thinking about casting Megan Fox into the role after seeing her in that photoshopped picture of her in the suit, but then I realized that there's likely better actresses for the role. After browsing, I found that Wilde has a good resemblance, and she should do fine in the role.
Green Lantern/Hal Jordan - Ryan Reynolds (X-Men Origins: Wolverine, Buried)
Yes, I just did cast this. Based on the few images and little footage of Green Lantern, Reynolds looks precisely like my idea of a live action Hal Jordan. I'd love him to reprise in a JLA movie.
Green Lantern/John Stewart - Chiwetel Ejiofor (Salt, 2012)
Will Smith was my first choice, but I think casting someone lesser known would be better. Ejiofor was good in 2012 and certainly looks like a capable John Stewart.
The Flash/Barry Allen - Chris Pine (Star Trek, Unstoppable)
Pine was rumored for the role for a while, and I'd absolutely love to see him play the Scarlet Speedster. He seemed to scream Flash to me when I saw Star Trek.
Martian Manhunter - CGI, voiced by James Earl Jones (Star Wars original trilogy)
Jones was the first one to pop into my head to voice Martian. Sure, it might seem weird for him to have a Darth Vader voice, but I can see it happening, to be honest.
Hawkgirl - Emma Stone (Easy A, Zombieland)
I'll admit, I haven't seen any of Stone's work beyond the trailer for Easy A, but she looks like she could kill the part. She also seems to have a bit of that Hawkgirl personality too, IMO.
Green Arrow - Ewan McGregor (Star Wars prequel trilogy, Robots)
I saw someone mention McGregor on an older JLA fan cast, and I must agree with his choice. Ewan is a good actor and he certainly could kill Green Arrow.
Black Canary - Kristen Bell (You Again, Astro Boy)
I admittedly don't know much about Canary at all, but Kristen Bell looks fit.
Plastic Man - Keanu Reeves (The Matrix trilogy, The Day The Earth Stood Still)
Reeves has been rumored to be playing Plastic Man, and that Warner Bros. wanted him in the role. He certainly looks the part, IMO.
Aquaman - Leonardo DiCaprio (Titanic, Inception)
He looks perfect.
THE VILLAINS:
Darkseid - Matt Damon (Invictus, Bourne trilogy)
This was rather accidental. I couldn't find anyone who could fit my idea of Darkseid, but I'm thinking Damon could do a decent job. He certainly has a similar face.
Lex Luthor - Mark Strong (Sherlock Holmes, Kick-Ass)
He's a good actor, and he should do fine here.
Catwoman/Selina Kyle - Anne Hathaway (Love & Other Drugs, Alice In Wonderland)
Hathaway was actually my first choice for the role, and was thrilled when she was confirmed to play Selina in . If Catwoman would ever be a villain in a JLA movie, Hathaway needs to reprise.
Ra's al Ghul - Liam Neeson (, Batman Begins)
I know he 'died' at the end of Batman Begins, but I'm pretty sure he is immortal in the comics (unless I'm off). Neeson was excellent in the role and I'd love to see him reprise it, if Ghul is still alive.
Two-Face/Harvey Dent - Aaron Eckhart (The Dark Knight, The Core)
I know he supposedly died too, at the end of TDK. But really...a long fall? The Joker survived a vat of acid that gave him his clown-like features, but Two-Face can't survive a simple fall? Unless he broke his neck or something, I can't see how he would be dead. Eckhart is perfect in the role.
The Scarecrow/Jonathan Crane - Cillian Murphy (Batman Begins, Red Eye)
Unless he gets killed in The Dark Knight Rises, I can easily see Crane being back for a future installment.
Poison Ivy - Christina Hendricks (Mad Men, All Star Superman)
I haven't heard of Hendricks until someone casted her as Poison Ivy before. She certainly looks like someone who can redeem the character (curse you Uma Thurman).
Well, there's my fan cast for the JLA. As said before, comments and suggestions are always welcome. Peace out!