Originally Posted by Mamba MVP
Not even have him in the film just tease at his presence like Nolan did with the card in BB. If they defeat Lex who else has that kind of cache with the public in for a JL film?
Outside of maybe Darkseid and Brainiac can't think of any others that would be worthy enough to require the entire JL to form.
That is actually a common question but there are plenty of threats out there that Supes cannot handle by himself. Darkseid and Brainiac as you said, doesn't need to be a big invasion angle either, it can work like Avengers did with Loki, Chitauri and Thanos in the shadows.
There are plenty of ways to twist and turns with different plots for those and even with Lex controlling the plan, he could do liek Tower of Babel or JL: Doom where Bats contingency plan were compromised, Lex gets it and hires all the baddies and target each heroes specifically. This is down the road though when the team is established.
Secret Society is like that too, a gathering of villains like JL.
Alien invasion like the white Martians.
Vandal Savage, another super smart person but he is immortal.
Amazo is a good threat, he can copy anyone's power. He can be a pawn and I wouldn't mind seeing this dude be the big threat Lex unleashes on Supes/Bats.
Solomon Grundy though I don't think they can make a whole movie just against him.
Felix Faust with Circe could work, all magic.
Anti-Monitor but that is like Galactus type of event.
Then outside of that they have a lot of b-list and C-list villains they can dispose of on the side or be that decoy for a larger villain like the Royal Flush gang or Queen Bee and the Hive.