A few thoughts:
On Cast:
Large casts of super powered people in a single movie don't work, because the audience not only needs to acquaint with their persona, but with their powerset, and if they don't have a common origin, their origin. There are ways around it:
1) Introduce all the characters and their origins in their own popular successful movies (Avengers)
2) Spread the core characters out over several films (Lord of the Rings)
3) Cut down the Cast (X-Men)
Now, 1 is not an option. Aquaman and Martian Manhunter are almost assuredly not going to get their own films. Flash and Wonder Woman probably won't either. That's 2-4 very serious origins and personas to go over and try to integrate meaningfully into the story or risk having the cartoon effect where 'anything can happen' regardless of if it makes sense. #2 seems doubtful. #3 is your best bet. Cut it down to about 5 leaguers for the first film, and grow from there if its successful. Assuming, the goal is to make a good successful movie rather than just an homage to the comics.
On Villains:
With such a large ensemble cast of intricate unique characters, you simply don't have time to introduce and define a bunch of villains. Without defining villains, you don't tell the audience who doesn't read comics what their limitations are, so they can't get 'into' the danger. It's kind of a random fest for them. That rules out ensemble villains like the Legion of Doom. Take a look at X-Men, with just four main X-Men, two of them were limited to basically supporting cast. Cyclops and Storm had few lines and no personal storyline or cool moments. The Brotherhood was composed of Magneto and, basically, three stock characters with no lines. They all had the same exact origin, no unique explanations, and their powers were simply what you saw, with simple themes.
Take Avengers, lots of heroes means one villain. In their case, Loki. I would suggest something similar for the League. Darkseid requires too much explanation and 'show not tell' to be better than Mongul, honestly. White Martians are a simple one-origin villain set that can be paired with Despero or something else like that. Alternately, to make things even more emotionally resonant, you can get an earth-based foe in the OMACs and Max Lord.
It's got to be big, because what happens in comics-- where Superheroes forget their full capabilities so that another superhero can shine -- doesn't work in movies. It makes the movie bad, but in comics, like a lot of crazy things, it's just normal.