In the film, Ferrell's supervillain imagines all his dastardly dreams coming true after defeating good-guy rival Metro Man (Pitt), only to find life quite boring without an adversary. To fill the void, Oobermind creates a new superhero, Titan (Hill), who also wants to be bad, forcing Oobermind to switch sides, Variety explains.
Tina Fey had already been onboard to play the reporter on the beat of the city's confusing superhero situation.
Downey Jr. is said to have had a scheduling conflict.
Separately, it was reported on Friday that Downey Jr. is in talks to star as Lestat in a Universal reboot of Anne Rice's Vampire Chronicles.