We talked about this before with Brainiac but

I think in order to distinguish themselves better from The Avengers, they arguably should have gone with a less extraterrestrial antagonist like the Injustice Gang or Legion of Doom.
Actually you know who would be amazing for a first go-around? Vandal Savage. He’s old enough for that he could have fought Atlanteans, Amazons, and who knows what else, so now Superman was “dead” he saw his chance to take over the world as he did in past personas as Alexander the Great, Genghis Khan, etc.
He then could have his lieutenant Deathstroke break Lex Luthor out of prison. They then team up for their own criminal counterpart to the Justice League.