While it is currently unclear as to what role Momoa would be playing, there is a little speculation that narrows it down. First up is Justice League member, the Martian Manhunter, who is a super-powered alien that is trapped on planet Earth. However, it has been long rumored that the films villain would be Doomsday, a scientifically engineered Kryptonian killing machine. Doomsday infamously "killed" Superman back in 1992, and his appearance was even hinted at in the extras on "Man of Steel" Blu-ray.
Known for roles that showcase his beefy physique, such as Khal Drogo ("Game of Thrones") and Conan ("Conan the Barbarian"), Momoa could fill either role quite well.
Neither Snyder or Warner Bros. had any comments on this news at the moment.