I agree, if a sequel is made, it's going to be more for the reasons why there is Cars 2 and Disney is prepping up Tron 3, they see franchise potential over the success of their predecessor films.
I think Green Lantern has a better chance than Superman Returns because even though Superman Returns is the stronger film, it really wasn't all that franchiseable. Many people saw Superman as a homewrecker, you really can't turn the supporting cast into merchandise, all you could really do was sell a very flawed Superman.
Green Lantern on the other hand has all sorts of action figures, a better video game, a hero that isn't as flawed as someone who has a bastard child being raised by another man, etc. He's easier to market to those little bastards we call children.