While I theoretically approve of the idea that a guy can create any weapon he can imagine and when he's dumb he makes a sword, and when he's smart he makes a 20,000 round a minute minigun, Green Lantern can't be made in to a good movie any more than Superman. Dude's just too powerful. Other than fighting an evil version of himself, who do you have him throw down with?
Clearly the answer is a non-corporeal planet-devouring entity because those always make the most captivating villains.