Combining rumors with already confirmed news, and just a dash of personal opinions, I've taken the liberty of this being the most realistic Justice League fancast on MP. Enjoy!
Clark Kent (Kal-El)/ Superman: Henry Cavill
This guy keeps it up, we could be looking at the best actor to ever play Superman. Obvious choice but had to be included for the sake of the article.
Bruce Wayne/Batman: Ben Affleck
He's going to knock it out of the park, relax naysayers...One of the 3 confirmed roles for the Justice League lineup.
Diana of Themyscira/Wonder Woman: Gal Gadot
She's currently in the process of gaining body mass and training in various skills such as jujitsu and swords. She can hold her own in the acting department and shouldn't be suddenly criticized in her role in F&F just because she was cast as Wonder Woman.
Enough with the obvious...let's get to all the rumors and speculations:
Victor Stone/Cyborg: Dwayne "The Rock" Johnson:
If he's not my personal choice (Lobo) then this is the next best thing. Couldn't see him as Black Adam for the sole reason of not hearing any noise of a Captain Marvel movie, so I'll reserve judgement. He has the perfect build for Cyborg all they need to do is tweak his origin story a bit. Other than that Dwayne Johnson = Cyborg.
J'onn J'onzz/Martian Manhunter: Jason Momoa:
Who knows what role Momoa will play if he's even in the movie but as far as a Justice League, my best bet is Martian Manhunter. He sort of has the look for it and CGI, makeup will only enhance it even more. He can handle the physicality side of things and can act well also. If not a future DC villain, I see Momoa as the Martian.
Green Lantern/John Stewart: Idris Elba
The most fancasted actor for a superhero role I've ever seen is Idris Elba as GLJS. But, it makes sense. At least the guy responded positively when a fan tweeted him a fan-made picture of Elba as Stewart. Denzel has been "rumored" although I think there is no truth to this whatsoever (the rumor came out on his birthday for Pete's sake). I'm sick of hearing Elba as Stewart, but it's the only one that makes sense as of RIGHT NOW.
Barry Allen/The Flash: Grant Gustin:
Including the Arrow-verse and The Flash-verse in the MOS world is 50/50 with me. The pros and cons of each are staggering but there is no other actor playing Flash at this moment than Grant Gustin. He's the only one to go off of so this is our best bet to play a JL Flash unless Warner Bros. decides to recast.
Oliver Queen/Green Arrow: Stephen Amell
Same with Gustin, it's not 100% confirmed that the TV and movie world will merge together or not but until then, we have Amell's Green Arrow to work with. Only time will tell!
Arthur Curry/Aquaman: Charlie Hunnam
No noise on a future Aquaman movie but since this DC Cinematic Universe is starting to get going, a film about the King of Atlantis could be near. Hunnam would be a great Aquaman (a great Green Arrow as well) this is my personal opinion as there have been no rumors of an Aquaman project.
Kara Zor-El/Supergirl: Margot Robbie
Has the look of Supergirl and she was good in WOWS. At 23, she's young enough to play the character for several movies. She IS Supergirl.
That's it for this fancast. In my opinion, this is the most realistic thus far combining confirmed news, long-standing rumors and speculations.