THIS ARTICLE CONTAINS SPOILERS! YOU HAVE BEEN WARNED!
Alright, so obviously theories have been flying back and forth since the movie came out, and its flown every direction from bloodlines to simple parentage, but I'm going to take things in a different direction. So this theory is based on one major plot hole, the prophecy. Anakin Skywalker never fulfills the prophecy. Does that mean the prophecy was never about him? Turning it over to his bloodline? Or maybe he still is the chosen one and is making an appearance in the movie you don't realize. So stay with me here, this is where things get a little far fetched, but its good, I promise.
So first, I want to start with Rey. Is it possible that the prophecy is about her? Of course now you have to be saying what happened to Anakin? Well, what if Rey is Anakin reincarnate and the prophecy passed to her? Stay with me here as we consider the evidence. One, she has extreme force powers, obvious by the numerous scenes where she overpowers a trained force user when she barely has any knowledge of the force at all. Two, Anakin's lightsaber calls out to her, but is it just possible that the lightsaber is calling to its old master? Also important is the fact that Anakin Skywalker retained his consciousness after death, and yet we see no sign of him after episode 6's closing seen, but we do see, or at least hear Obi-Wan Kenobi, Anakin Skywalker's master, calling out to Rey/the reincarnate Anakin.
Ok, so, now that I've given evidence for that, here's something else that begins to become a sparkling possibility. What if Luke is the reincarnation of Qui-Gon Jinn? I know, even a further stretch, but think about it. Anakin's first master comes back to redeem Anakin to the light side to allow Anakin to retain his consciousness after death, then allowing Anakin to later be reincarnated and find his first master to be trained once more as it should have been. Consider, Luke and Qui-Gon's lightsabers are both green, a small note, but true. Next, both are connected to Anakin through the force in some powerful way. When Anakin kills the sandpeople in episode II Qui-Gon's voice is heard calling out. When Rey is trying to sleep she imagines (or is given a vision of) an island that she recognizes at the end of the movie as the island Luke is on. A powerful force connection to the reincarnated as well. And lets not even mention the whole "Luke, I am your father" thing.
So, recap, Luke is Qui-Gon reincarnate, Rey is Anakin reincarnate, they are both there to complete what was started in episode one, training the chosen one to bring balance to the force!