The original Phoenix Saga would not translate directly to film. The emergence of Phoenix, her saving the universe in the M'Krann Crystal, her dealing with her new powers siwtching on and off due to psychic circuit breakers, her slow corruption by Frost/Mastermind, her turning into Dark Phoenix and flying off in to space and devouring a sun - thus killing a planet of billions, her return to earth, her defeat by Xavier, the battle on the moon resulting in her sacrifice, and her return years later from a cocoon in the bay - all that took place over many, many years of comicbook issues. It would never work in one movie, not even its own trilogy.