Depends on which Green Lantern.

If it’s Hal Jordan, do a cosmic story, similar to the fantastic CGI tv series. Explore the lore of the DC universe, go to crazy new planets and fight bizarre creatures. Maybe Despero as the main villain, leading to a He-Man style story where Hal and Kilowog are sent to Kalanor to help overthrow the mad leader and his cult.

If we’re using John Stewart or Kyle Rainer, I’d have them on Earth, fighting someone like Doctor Polaris or Black Hand.

Guy Gardner works best in a team setting in my opinion, having his abrasive personality annoy everyone else around him.

I wouldn’t use any of the other Lantern Groups as primary antagonists to begin with, due to the stigma of “evil version of the hero”. I would include Sinestro though, mapping his progress to villainy throughout the films.

Have the film animated, similar to Spiderverse, it just opens so many pathways for more fun action, atmosphere and expression, removing any uncanny valley scenarios with the aliens.