I think that if DC wants to make a shared movie universe like Marvel they should make the introduction of Justice League as a trilogy, well, not exactly a trilogy as all movies would stand on their oun and only the last would be a JL movie.
The first would be World's Finest, to make a team-up between Superman and Batman, and to show that both characters can coexist in the same universe, next would be Trinity, to show that magic exists in thiss DC universe, with the movie being a team-up between Superman, Batman and Wonder Woman, maybe fighting Ras-All Ghoul like in the Trinity 4 issues mini-series, having him use Bizarro and Artemis or another Wonder Woman villain.
Then there goes the Justice League movie, maybe have Darkseid attack? If not there are other cosmic threats in the DC universe that can be used, i think that if they instead decided to use the JL movie as way to introduce Flash, WW and other characters then it should be the original line-up without Superman and Batman, they didn't even appear much in the original JL comics, and i think it would still work as Flash and WW are popular in their oun right.