Ron Marz stated that he was in the process of bringing the Corps back near the end of his run, which would have seen the Corps rebooted into a smaller group led by John Stewart and Ganthet as the Guardians. Unfortunatly his contract ran out, Judd Winick went another way and we had to wait till Rebirth to get a corps back.
Also by Marz was the elseworlds Green Lantern: Camelot in which The ENTIRE cast of Green Lantern would take place in the time of Camelot, with Alan Scott as King Arthur, Hal Jordan as Lancelot, and Ganthet as the Lady in the Lake (Marz: He has a dress!)
After losing his yellow Power Ring and after the Zero Hour crossover, then-current writer Beau Smith wanted to revamp Guy Gardner as an Indiana Jones-type hero (i.e., without his powers), but he was forced by editorial to add the Vuldarian shape-shifting powers. You can learn more here.
Box Office Bomb: Despite having a strong opening weekend, the box office gross ultimately tanked.
Creator Backlash: Perhaps bordering on Old Shame. Ryan Reynolds indicates that he does not consider making the film a positive experience, and that he has no interest in starring in a future Justice League film unless assured that the script and director are of high quality.
Executive Meddling: Allegedly, but if you have seen the movie it does seem like many scenes were trimmed or cut, and it is known that some scenes were re-shot after production wrapped up. There is an extended version that adds a few minutes, but still leaves out a lot according to at least one source.
One thing I feel needs mentioning: this is not Martin Campbell’s cut of the film, but the studio’s. I live in New Orleans where it was shot, I read the shooting script, all of which was painstakingly filmed with intense research, and all of that was left on the cutting room floor — a sort of combination of what happened to Daredevil and Watchmen, respectively — character development sacrificed for CG, scenes made irrelevant by removing their setup. The movie in the theater starts with an explanation of mythos that is made redundant by the more natural, scripted questions from Hal when he gets the ring. Ten minutes of childhood Hal, Carol, and Hector that sets up Hal’s first ring construct is reduced to an awkwardly placed flashback in the middle of another scene. The training with the ring is almost completely excised except for one minor scene. Most appallingly, the ending completely deletes the fact that Kilowog, Sinestro, and Toma-Re arrive at the end and help Hal defeat Parallax. Not to mention Parallax was supposed to be a 3rd act reveal after we spend the film worried about Hammond going evil, not the main villain for the entire film. I sincerely hope we get a director’s cut or at least all the deleted scenes on the video release.
Originally they were gonna use the villain Legion, until they changed it to Parallax.
Until they decided to make Hal definitively the first human Lantern in this continuity they had plans on revealing Alan Scott as one of the government agents and a former Lantern, more-or-less replaced by Amanda Waller in the film proper.
They originally intended to make a practical Green Lantern costume to be enhanced with CG but the more they experimented with the technology they knew they needed to make it fully CG, they didn't want to show audiences another rubber/spandex/leather superhero costume. But there is concept art for a more Batman-style armored GL look.
This film was going to launch the DC Cinematic Universe until its horrible reception from fans. This certainly helps explain its only containing the barest hints of foreshadowing for Sinestro, as well as Amanda Waller showing up despite having no impact on the story.