I wouldn’t release it until after a new Supes makes his big screen appearance (either in a solo film or as a supporting character) and after the GL series had finished a season or two. Basically when the solo franchises are firing on all cylinders.
Call it The Justice League, and move far away from the style of the first movie. Meaning no muddy red skies, and no more action scenes in grey cylinder-shaped indoor environments, for the love of god, and no more JL meetings in Batcaves or Bat vehicles. The League is bigger than that.
As for the lineup, I would start it with Supes, Bats, WW, Flash, GL (John, the newest member, stepping in for Hal), Aquaman, and Cyborg, with Hawkgirl being recruited at the start (our wide-eyed POV character, as she's given a tour of the Watchtower). The mysterious Martian Manhunter is a focal point of the narrative, and Shazam is a last minute surprise to deal with Black Adam.
Basically this:
Secrets and trust would be a major theme, with Bats’ contingencies (Tower of Babel story), their secret identities (especially Billy's), Martian Manhunter not trusting the league, Bruce’s real intentions for recruiting a telepath, etc, all contrasted with...the Legion of Doom, made up of...
Lex, Joker, Cheetah, Grodd, Black Manta, Deathstroke, and Black Adam.
But Grodd will also use mind-control on different League members to even the playing field. So we can get a Flash/Superman chase, a Hawkgirl/GL couple fight, a savage Aquaman/WW brawl, etc.
And Martian Manhunter, with his own mind powers and connection to the others’ minds would be instrumental in both the Legion’s plan, but also how the League save the day. Maybe he helps uncover some of the Legion’s own secrets, how they actually all have their own secret endgame (or at least Joker does), and this causes the Legion to fall.