IMO this can only be done with another movie like Hulk or Supes do.
I think the first thing they need to do is change Hal for John or Kyle as the main GL. John being one of the most well know African American superheroes, his casual fanbase is probably the largest. Kyle in the other hand is DC Spider-Man, he can cover the youth audience. As for the genre it should be like Predators (hell, they could even include some Predators or Aliens as cameos, GLs have actually fought against them in comics) Also, the Avengers just proved team superheroes movies = $. They should include all 4 GLs and call it Green lantern Corps. And make of it the best cosmic superhero movie.