Writer/Director:
-Christopher McQuarie (MI: Rogue Nation). JJ jumped from MI3 to Star Trek just fine. I think McQuarie could make a similar jump. Plenty of experience writing soldiers and sci-fi, and he directs thrilling suspenseful action.
Tone:
-Very End of Watch, District 9, Training Day, Edge of Tomorrow with regards to the Corps, but mixed with that more magical adventure tone of Star Trek & Star Wars once we're introduced to the mysterious emotional spectrum.
-Utilize familiar law enforcement / armed forces visual motifs with Oa and the Corps to make the GLs more digestible to audiences who've written off GL as something random and cheesy, but still have the GLs go to visually majestic worlds like Zamaron with completely foreign lifeforms to bring that sense of wonder and creativity. It's something the other sci-fi franchises don't seem to utilize enough.
Some images that sort of capture the vibe I'm picturing:
Plot:
In an opening battle scene, Hal's partner Guy is injured, so his ring finds a replacement, John. Hal shows John the ropes, and a bulk of the first two acts deals with the day to day life of a GL, resolving simple disputes, interrogations, chases, etc, with the mystery of the yellow lantern corps unfolding. Hal and John butt heads over a lot of issues, including Sinestro's proposal to drop the restrictions on lethal force, and perhaps Hal's unwillingness to believe John's accusations about Sinestro.
The third act would involve the GLs on an enslaved Korugar, and subsequently a battle between the two corps while Hal takes on Sinestro. Conclude with Hal being promoted to Honor Guard, Sinestro getting what he wanted with the GLCorp dropping the lethal force restrictions, a hint of Hal being infected, and John meeting his new, even more loudmouthed partner Guy.
Only show Earth in one scene at the start where John is chosen, and perhaps another brief one before the third act where Hal or John visits the other's home, gaining a key understanding of that character's past.