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Spoiler...According to the details of the script, Guy Gardner and Clark Kent (no word on who is playing Kal-El) both have cameos; and Legion is fought by Hal and the Corps in a "climatic" battle, which gains him the respect of the Guardians, leading him to become 2814's Green Lantern. But Sinestro is supposed to have a story-arc in there, too. No real elaboration on that detail other than its possible we don't get every bit of Sinestro in one movie. So, I guess we won't really know until the project progresses.