I would have chosen Fillion as either Hal Jordan or Alan Scott
At 52, I think having Nathan as Hal, who is generally the most iconic GL, would be limiting how long he could play the role convincingly resembling the comics Hal. They’re definitely going to go for someone 20-30’s, maybe early 40’s when casting Hal. He would have been great as Hal, but the time has passed IMHO.
He might have been fine as Alan, but I could him nailing Guy’s brand of asshole-ish humor. Plus, Alan’s a tricky one to handle since usually his origin is totally unrelated to the rest of the GL Corps.
Since Superman: Legacy won’t be an origin film, I assume Superman may even have some scenes of him traveling in space, so I wonder if he’ll encounter Guy and/or Hawkgirl there or on Earth.