I think that the costume should have a bring to mind several things, the suit should be spacey and futuristic feeling. Not to say that it should look like the stereotypical future space suits. But it should give a feeling of futuristic and spacey. Maybe be sort of shiny or glimmery and form fitting like the spider-man suit, and not made of rubber or leather. It should look like an alien fabric, or an unearthly (because he made it with his ring and imagination) fabric. Maybe it can be some what reflective, not really reflective or like plastic looking, more like very subtle reflecting. Almost not at all. It should also give off a cop or law enforcment feel, because thats what Green Lantern is, a space cop.
Just stick to the comic book design, but make it like Spier-man's from the movie.
I think they should go the Spider-Man rout, keep it like the comicsa design (Hal Jordan design) but add details and stuff to make it work in real life. I like the idea of little patterns, sort of like Spider-Man's suit (when you get in close you can see cell looking designs or squars)
or Superman Returns. Maybe they could subtelly put in Green Lantern symbols as a pattern in his suit (so subtle that you can't really notice them.)
As for his symbol I think it should be almost the same as the comic only like Superman or Spider-Man a little 3D, only toned down more than Superman's.
His gloves and his boots should be almost inseperable looking, like they are all connected, like Spider-Man's suit. Seemless, like in the comics. No rinckels or material hanging off. I do like the metallic looking parts too. Plus, this uniform is created by the ring and Hal's imagination. So it would be like Spider-Man's (fit like a glove with no seems and no rinkles). And have lots and intracet designs and stuff to make it look spacey and something that came from someone's mind. I guess I wouldn't really add or take anything away either, keep it as close to the comic as possible.
His ring should look like the comic version only sort of glowy. Like it radiates energy, especially during use.
Out of his GL uniform, because its a cop uniform for the space cops the Green Lanterns, Hal wears a pilots bomber jacket, plad print shirt (maybe with hints of green and black, and silver, jeans and boots.
Just DO NOT make is costume look rubbery, or have artificial looking muscles, or anything like these costumes: