I don't think The Spectre will be anywhere near this franchise. I definitely don't want to see Hal becoming Parallax on the scale he did in the comics, but I wouldn't mind seeing him succumb to a lust for power, grief, etc. That story revealed some powerful ideas about great power and the burdens of it, and it could be used to fantastic effect. I say have Hal lose it after Sinestro destroys Coast City (or appears to) in an act of revenge, and very much go down the Parallax trail. Have Kyle Rayner be the Lantern who is tasked to help stop him, with Hal ultimately turning away from the dark side, but going off into space to "find himself" at the end of the film.