I hear you -- what would they even adapt from the comics, if they tried?
Even though the X-Men movies have been awful (IMO), at least the stuff they've been adapting from have been good-to-great comics.
Same with Marvel.
But like you, I have absolutely no faith in a Justice League movie. I just...there's just no reason to believe it would be good. You can barely find source material that's consistently good.
If you treated them as what they are, namely 3 of the most popular and widely-known characters in all of fiction, 3 other guys who most of the world is at least familiar with thans to the Super Friends, and a Martian, and just open cod with them already as a team, you could tell damn near any story you want. Do Morrison's white Martians arc (or Timm/Dini's JLAU white Martians story). Hell, do Rock of Ages if you want. Just assume your audience knows the heroes and go nuts.
Bt no, you know they'll have to do an "origin" story and introduce everybody, good and bad, and probably include Darkseid b/c that's what Johns just did, even though that character by himself is dull without the rest ofte surrounding mythology.