I think a JL movie is attainable, they just need to go about it the right way. If that means copying Marvel's Avengers model of introducing characters in solo films and then putting them all in one big picture, so be it. I know they said that a JL film would be "separate" and consists of new actors playing the same roles, but I don't know if I buy that. I think that might be true for Batman, since Nolan's Batman is set in a world (apparently) without superpowered beings. So a more comic book-esque Batman might be appropriate for JL.
GL is also a wild card here, since the first film didn't perform like they'd hoped and the reception was... not the best. However, this could be the perfect opportunity to get people on board with GL. If they include him in a Justice League movie and the response is positive, that might get people excited about a GL sequel where we see Sinestro's downfall and all of that. Of course, they could also decide to either recast the Hal Jordan character (likely with a lesser known but still dependable actor like Nathan Fillion) or move on to John Stewart.
I'm going to say this right here and now: if MOS is a hit, then there is no way in hell Henry Cavill won't be playing Superman in a JL movie. The only way it wouldn't happen is if 1. a JL movie simply isn't made, or 2. WB stupidly decides to leave Supes out of a JL movie. But if it does happen, WB won't be stupid enough to cast yet another actor in a role that is so difficult to cast in the first place, especially if Henry is a hit with fans and critics.
That brings us to other characters. I know there are plans for Hawkman, Flash and maybe Wonder Woman and Aquaman. But these characters, IMO, should be introduced in a JL film. It just feels less risky. After the GA has been properly introduced to them, THEN spin them off into their own movies and see if audiences follow. Why do so many of us that grew up in the 80s know Green Lantern, Hawkman and so many of DC's more obscure characters? We read the comics, sure... but our original introduction to these characters was SUPER FRIENDS. Cheesy? Sure. But there was something cool about seeing ALL of the heroes together. I expect Avengers to be a massive blockbuster, and a Justice League film would be the same.
Some ideas for a JL cast... this is assuming they use a new Batman continuity, separate fron Nolan's. Also, not sure which Flash they'd use, so I posted ideas for both.
Superman: Henry Cavill
Batman: Timothy Olyphant
Wonder Woman: Christina Hendricks
GL Hal Jordan: Ryan Reynolds or Nathan Fillion
GL John Stewart: Shemar Moore
The Flash, Barry Allen: Andrew Lincoln
The Flash, Wally West: Mark Paul Gosselaar
Aquaman: Ryan Phillippe
Martian Manhunter: Lance Reddick
Hawkman: Jason Momoa
Hawkgirl: Diane Kruger
Captain Marvel: Owain Yeoman
Green Arrow: Max Brown
Dr. Fate: Hans Matheson
Amanda Waller: Angela Bassett
Max Lord: Bradley Cooper