I think basically Iron Man is going to save this Franchise. I don't know if The Green Lantern is involved, but Thor and Captain America are not that big of draw either. If this movie does good then all of these other movies will do much better the next time around as more non-super hero comic fans will be participating in going to see it. Who here plans on watching it when it comes out?
As others have said, The Green Lantern is nor part of the Avengers, he's DC whereas The Avengers are Marvel, lol, so unless they did some weird crossover Marvel vs DC film, we'd never see them together, the other member you're missing is The Hulk, lol.
I'm planning on watching it when it comes out, and will see Captain America when that's out here too, I've seen both Iron Man films, both Hulk Films (Although they're using a different actor anyway... they need to find an actor that they can keep on instead of changing each film...) but not Thor yet, Thor doesn't really interest me at all, while the others do.