So far, most of Warner Bros.' films based on DC Comics characters haven't had the kinds of ending credits sequences that are staples of Marvel's movies. That all changes with Justice League, with a major hint about a future challenge to the team's heroes.
Once all the names finish rolling down the screen, the film cuts to a scene in what's presumably Arkham Asylum. Guards are taking a headcount on prisoners, with one yelling at Lex Luthor to stop dawdling. Shown from behind, Lex doesn't budge. A guard goes into the cell, puts a hand on the genius' shoulder and spins him around, only to find that it's another bald white guy.
We then cut to a speedboat heading to a luxurious yacht. A close-up on the back of a character's head shows two strands of fabric flapping in the wind. Yep, it's Deathstroke, who we next see taking his mask off and talking to Lex about his payment for springing the criminal mastermind out of the pokey. Lex makes some comments about the events of the movie, snarkily noting how the heroes have all teamed up. Luthor then essentially says that he, Deathstroke, and otherlike-minded individuals should form "a league of our own".
So, yes, it seems like the DCEU's bad guys will be forming their own worse-together unit sometime in the future. In the video at the top of this post, I talk about the two primary bad-guy megateams in DC Comics' publishing history and speculate about who might show up on Lex's villainous conglomerate.