So I just saw the Force Awakens again and I was thinking of something. What if we see Kylo Ren rise to the light, but Rey fall to the dark? Her first time using the force in combat is just after Han's death, and she's DEFINITELY pissed at Kylo Ren. I really believe that she came close to using the darkside during the fight with Kylo. She was upset and angry and seemed to draw on that to fight. It's possible, especially if Finn is also force sensitive, that we could see Rey fall to the darkside, and Kylo get redeemed and come back to the light. And the two of them would have their roles reversed in a way in that it would then be Kylo bringing Rey back to the light herself. I really doubt this is actually going to happen, but it is definitely a road they could go down that would lend some surprise and unexpected character development in rest of the trilogy.