There can be only one villainous threat considerable enough to really unite the JLA under one banner, and that is Darkseid, the Apokoliptian scourge of Superman inspired by Hitler with a typically voracious thirst for destruction, who Bryan Singer initially wanted for the Superman Returns sequel. Single villains, or super-villain teams (as the JLA have faced frequently in the comic book canon) can offer a stopgap, and particularly diverting ones in the case of Prometheus, but Darkseid is to the JLA what Thanos is to the Avengers, and what Magneto is to the X-Men. He is the single most enduring villain of that universe, and he carries a significant enough threat to ensure that all of the JLA members are tested simultaneously. The Rock has attracted numerous rumours since it was revealed that he was in talks for a DC movie, and while Black Adam remains the most enduring possibility for him, he has the charisma and the presence to pull off a considerable villain role like Darkseid. Johnson would channel some of the free-flowing charm that he used in building The Rock's character, and would ensure a complexity beyond what most fans might expect of the Apokoliptian tyrant.