I've been trying to avoid anything Star Wars themed because I don't want to read any spoilers, so someone else may have come up with this, but ... the other day I was just thinking, "I wonder what kind of curve balls J.J. Abrams might throw at us to make Episode VII unique and put his stamp on it?"
He took some liberties in Star Trek to do just that, like the Uhura/Spock romance, Kirk dying in a mirror of Spock's death, etc. And it hit me -- "what if Abrams has Luke Skywalker as the mysterious not-Sith overlord of the Knights of Ren?" Now that would be a twist-and-a-half, but it would also be somewhat fitting since Luke did unleash a fair bit of Dark-side like rage in Return of the Jedi between force-choking a Gamorrian Guard and laying a beat-down on his father.
I also noticed that in the previews, Luke is the only one of the old heroes who we haven't really seen yet. There is a fair bit of Han and Chewie, and a brief shot of Leia. You do see Look putting his hand on R2-D2, but you don't see his face, and he's covered in a black cloak.
I'm pretty sure I'm dead wrong on this, but it is a fun exercise to think about how shocking and unexpected it would be to have Luke's fall to the Dark Side be a key piece of what has kept the not-Empire (First Order) going so strong. That's probably what I would have considered doing if they'd asked me to write Episode VII.