Have the first third of the film on Earth. Build Hal up as a character, HINT at his relationship with Carol, have more scenes with his family.
Then once he gets the ring and goes to Oa he should stay out there, in space, for the rest of the movie. Have him go through numerous training sessions. Show him grow and learn as a person, as he grows and learns as a Lantern. He starts off as a fearful rookie who is resented by Sinestro, but show him progress. Maybe even going out with other Lanterns to see how they do their things in their sectors. Maybe a fight scene with some strange alien or two. Show the relationship between him and Sinestro go from cold mentor/student, to genuine friendship by the end. Show Hal learning by himself that you don't have to be completely fearless. But being able to recognise fear, confront it, and defeat it. Have him learn that by himself as his time in space goes on, instead of having Carol spoon feed it to him.
Have the Manhunters be the big threat of the movie. With the grande finale an all out attack on Oa, where the rookies, including Hal, have to step up to fight.