2. Darkseid - Dwayne "The Rock" Johnson
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.