5. Adam Driver As Luke Skywalker's Fallen Apprentice
We've all but known for a while that Adam Driver would be playing a Sith-style villain in the new trilogy and establishing him in the first film is a good thing; even though Palpatine was the central threat in the prequel trilogy, there was the distinct lack of kick-ass villain like Darth Vader across the films. We need a baddie that can be established as a threat across the entire trilogy. Let's assume that Luke found his first apprentice just a few short years after Darth Vader and Palpatine were defeated. That apprentice became his biggest success and was the driving force in other potential Jedi joining his academy. That apprentice is now a fully-fledged Jedi knight twenty-five years on and training other Jedi. Now what would happen if Luke's greatest ally turned to the dark side? That would be an interesting question posed by the film as Lukes good work is undone. A man with great power, perhaps equal to Luke's own and with his own apprentices, succumbs to greed, anger and power and forms a new academy; the return of the dreaded Sith. It would allow for some epic Jedi versus Sith battles later on in the trilogy and be a tragic retelling of the Obi-Wan / Anakin story.