I reckon there must be some kind of boot camp sequence. Green Lanterns are cops, not Super Heroes, so it'd make sense to have the ensemble largely made of rookie Lanterns to Train alongside Hal.
Kiliwog, Salaak and Bzzd are the most likely to be 'main' characters. Kiliwog is the Lantern's Drill Sergeant so he has to be in a fair bit, and Salaak is the most senior Lantern in The Corps. Bzzd is partner to Mogo, the living planet so I imagine if they include Mogo as a location, Bzzd will have a fairly large part in any sequence there.
Isamot is, I think, the partner to Abin Sur, Hal's predecessor who passes on the ring. He's also likely to be a proper character.
Then you have Sinestro, obviously.
A lot of characters. The rest will, i think, be background.