Was bored, so I did a fan-casting for the movie:
Superman - Henry Cavill
No-brainer, really, if you want JL to share a universe with MoS.
Batman - Tahmoh Penikett
With this kind of jaw he has to play a superhero at least once, and he's got the physique and acting chops for the less semi-realistic Batman needed for a JL movie.
Wonder Woman - Morena Baccarin
Yes, she's over thirty, but she's still my favorite candidate for the role, and since this is my wishful thinking, anyway, I say, screw the age limit some have put on WW.
GL John Stewart - Chiwetel Ejiofor
He's just a damned good actor, period.
Flash Barry Allen - Simon Baker
Baker mostly has the likability and the mature lightness associated with Barry.
Aquaman - Nikolaj Coster-Waldau
I'm seriously surprised he's not on everybody's list. He's got the looks, the acting abilities and the charisma. What else could anyone ask?
Martian Manhunter - Carel Struycken
At age 64 he might be a bit too old, but dammit, I've yet to find another actor this suited for the character. He's incredibly tall, relatively thin, got the brooding part down, and, seriously, look at him. Just paint him green and give him red lenses, no more make-up needed. And as for the age part, with J'onn's shape-shifting powers, the action would be mostly cgi, anyway.
Despero - Keith David
Yes, I'd pass on Darkseid for the first movie, the JL has more good villains, and Despero is especially interesting with his mind-controlling powers. He's also a galactic emperor, so he could keep all the heroes busy. As for the casting, another no-brainer. David has voiced Despero on the JL cartoon and will reprise the role on Young Justice. He's a great actor with a fantastic voice. Why look anywhere else.
Amanda Waller - Kathy Bates
Waller has to be intimidating, and there's no actress who can do that quite like Bates.