I think we're all making everything more complicated than it is. I can only see them doing Justice League in the 2020s. In about 10 years from now.
You have to look at where all marvel and dc movies might be next decade. Let's start with Marvel just humour me for a sec. The way i see it, All the X-Men prequels will be done. The next 3 or 4 Spidey movies will be as well. Which means they'll either move on from Marc Webb or Andrew Garfield by then (or both). Downey wont be doing anymore Iron Man flicks, we'll get 2 Avengers this decade, by 2019/20 maybe a 3rd will take place. I dont see them going over that. (It'll be hard enough getting Downey back for a second AA or a fourth IM). They also won't rush into rebooting and give the fans a different Tony Stark. Caps/Thor sequels will be wrapped up. Plus the one-off Wolverine flick will be complete & a possible Daredevil reboot by the end of this decade.
OK, now. DC...Snyders Superman trilogy, Nolans final Batman, a good 2 movies from the Batman reboot. ALL RELEASED IN THE NEXT 8 YEARS. They're certainly taking their sweet ass time with Green Lantern 2.
With that said, what's my point? The next decade is going to be ruled by DC and they'll want to surround its characters around debuting a JLA. JUST like this decade is big with The Avengers. They'll launch Flash, Aquaman, Wonder Woman & a new Green Lantern no doubt about it. They'll continue pumping out more of these "Bat reboot" solos and id like to think the fans will be won over by this version enough to include him in Justice League with a new Supes. Cavill will be something of the past just like Bale will be in a couple years.
Is Marvel the type to possibly try to bank on Ruffalos Hulk (non origin this time)? or to try a different Iron Man? It's possible. But they'll go under the radar after one movie. Downeys the money grab for Iron Man, its the only reason the character is where it is. Fact is Marvels prize in the 2020s will be Spider-Man. He may even stand alone. DC will make the best of this and Justice League will be launched with a new Superman, new Batman, and its other heroes.
Sometimes you have to just see something at face value, if Zack Snyder says his Superman is seperate from a future Justice League Superman, then he's probably right. They'll wait for this version to come to a close. Cavill will probably be loyal to Snyder as Bale is to Nolan. His training is also intense & u gotta believe he'll want to leave the franchise after 3 movies and not continue with such a demanding physical role. A Justice League will ensure sequels & multiple cameos.
Sorry for the long winded post, but after reading the last couple of pages, i think we're getting confused by mixing multiple Supermans, Batmans. It's just not how it'll happen. They're smart and patient, they'll wait til the next bulk is over with.