Not really sure what the budget has to do with anything. Just because GL may not look like the 200 million movie it is, doesn't affect my enjoyment in any way.
It didn't actually cost 200 million, but that's kind of a moot point because a lot of other films don't actually cost as much as is claimed either.
I really don't see how they could have done much differently and been as faithful to the Green Lantern comics and mythos as they were.
I think they could've have simplified the story for one. Wasn't it in the comics issues and issues before Hal went off planet? Maybe the story could have been ALL Earth based and at the very end the Corps arrive... showing Hal a whole big universe, setting up the sequel.
I could've been all SPACE based. That would've been different for a superhero movie. Hal, fearless, yet self-involved, gets the ring, flies off into space, trains with Sinestro, they do a mission together, someone self-involved Hal has to save people he's never met, he changes, and flies back home... a changed man...