Legion is such a bad character. I don't think he adds anything. The mentally ill character who is sometimes a friend and sometimes a menace has been done before. And the different powers for each personality is rather trite. He also doesn't work as Professor X's son. Thematically, Professor X and Cyclops is father and son, and it works well. Professor X being in a wheelchair and perhaps incapable of having children, and Cyclops being an orphan. Introducing Corsair and Legion undermines that.