By the end of Justice League, both Bruce Wayne and Lex Luthor are putting together their teams, and while neither one is outright named, the implications are obvious. Bruce is thinking of a table with six chairs… or maybe more, depending on how many super-people respond to his RSVP. It's an obvious nod to the Hall of Justice, which pretty consistently features a table at which all of the members of the League make all of the big decisions, like how they're going to beat the newest evil, or if they should order from Quiznos or Subway.
Meanwhile, an escaped Lex Luthor talks to Deathstroke about forming their own group—which, per DC Comics tradition, can only refer to the Injustice League, a villainous collective founded by Lex Luthor that at one point counted Deathstroke as one of its members. Since there have been at least 60 villains who've cycled in and out of the Injustice League, it doesn't really drop any reliable hints in terms of the future of DCEU villainy… but was that thief being apprehended by Wonder Woman at the end of the film possibly Selina Kyle? Black Manta is scheduled to be a villain in Aquaman's solo film, and half of the Suicide Squad have been members, so that's half of a League right there.