Like Ronny says, Hal shouldn't be pre-ordained to be the greatest GL ever. I think we've seen enough stories about "The One", in various forms. He should just be a guy who establishes himself as one of the primary assets to the Corps over time, through hard work, rather than have him fulfilling some kind of prophecy or anything.
Also, I don't think Sinestro should be behind Legion. He should be a firm believer in the nobility of the Green Lantern Corps. Being a Green Lantern is his life, his one source of happiness. He shouldn't hate The Guardians and already be plotting against them. His sins come from a desire to maintain his reputation as the greatest of the GLs.
The way I'd personally do Sinestro's turn to evil, would be with a "Gone Baby Gone" style moral conundrum. Throughout the movie, you built up Sinestro as first Hal's mentor, then his friend. You make him into a nuanced, almost likeable character. And all along, we here about how orderly his sector is, etc etc.
Then, either at the end of the first film or in the sequel (depending on when they pull the trigger), we have Hal discover that Sinestro rules over his sector with an iron fist, opposing a fearsome, fascist rule over all. When he discovers this, Sinestro pleads with him, tries to reason with him. He cares for these people, he's acting in their best interest, it seems like a hard regime, but it's all they know, and was necessary to stop them from killing each other. If Sinestro gives up his dictator-like rule, this sector will descend into violent anarchy and civil war, just like the way things were before Sinestro took over. He begs Hal, as a friend, not to betray him, because the Corps is everything to him. And so Hal must choose between what's right (in a strictly legal sense, adhering to the code set out by the Guardians) and what is right (in terms of standing by his friend, helping him act in what he knows is the greater good).
And ultimately, Hal turns Sinestro over to The Guardians. Sinestro is removed from the Corps, and exiled to Quard. He loses everything, and so swears revenge on Hal.
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