Superman, Clark Kent - Henry Cavill
It's been done. He's a great actor, really thorough, and he fits the part. Really no reason to change this. I don't even need to list the pictures, because we've all seen him AS Superman at this point.
Bruce Wayne, Batman - Karl Urban I just kinda saw this yesterday and it hit me well. Somewhere after Chronicles of Riddick he became this great actor, and he has the darkness the brooding, the action experience, the whole thing. He can do this.
Diana, Wonder Woman - Jamie Alexander Jamie's not the most accomplished actress, nor is she the ideal women that some feel is the only one good enough to play WW, but she has several really important things going for her: 1) She's believable hitting a grown man and knocking them down. No matter how much story super strength you give her, if you have a stick, it looks odd. 2) She's got experience with action and riggs and so on and so forth. 3) She loves the character. Period. 4) She looks like the character. 5) She's up and coming, with films like Loosies and Savannah, people know she can act very well, even if we haven't seen a lot of yet. Very little of this can be said of the 5'11 models and the busty tv stars that are often fan-cast.
Green Lantern John Stewart - Anthony Mackie We've heard this name before for him, but it's a really solid casting. He's really talented, from the Hurt Locker to Notorius even way back in 8 Mile, and is one standout role away from being a big star. A sort of restart of the GL franchise, with him at the lead, could do very, very well. There's a lot of fan cry for the original 7, but I don't think you'll see a lot of the general clamoring to see Reynolds' Hal Jordan again. A new Lantern might be just what the doctor ordered.
Barry Allen, The Flash - Jake Gyllenhall Again, another near A-lister, and for good reason. The idea is that the films leading up to Justice League would be stellar. Gyllenhall can carry a movie on his own, we all know that well. It'd be good to see him to it here.
THAT'S IT. The problem with a JL movie is that when you have more than 5 main heroes, you end up with either a villain with no development, or
Now, as much as I'd LOVE to see an alien invasion film, it's really hard to argue the simplicity and the usefulness of an OMAC-fueled Max Lord storyline here. This gives us two distinct advantages. 1) It's easier to explain/suspend belief for as an Earth-tech-based thing, which is important with so many high concept characters gadding about. 2) It provides only one villain that needs development, but that can be developed sufficiently. The power of an alien storyline would be the mind control turning heroes against each other. Max Lord can do that. The coolness of Batman resides in being equipped to take down the other heroes. Brother Eye addresses that issue. And with plenty of troops out in the open, it not only gives enough fronts for the entire league to battle on (some go after OMACs facility, some after Brother Eye, some after Max Lord), but it also gives a precedent for a superhuman protective force, that explains why the world, logistically, needs the League.
OMACs/Brother Eye - Voiced by Frank Welker He can voice any and everything. Let it happen. In this story, the OMACs would be the canon fodder villain that starts problems for the beginning, and paints Batman as the villain at first.
Maxwell Lord - Jim Caviezel This is the main bad guy, and he has to be psychologically able to contest all the heroes, if this is truly to be an ensemble piece, and not just a Superman-Batman movie sequel. You need someone who really has heft to them, and I'm not sure Michael C Hall can handle that. He could barely own the screen against Gerard Butler in Gamer. Having him fund the JL at first and play both sides against the middle could make him a fascinating villain.
That said, I would probably cameo other heroes, as those who choose not to help found the league for whatever reason. This way you can set up the breadth of the universe, but without having to explain every little thing.