I mentioned before that I thought Rey could be Luke Skywalker’s daughter, and there are theories that she could be Obi-Wan Kenobi’s granddaughter. There were hints of Obi-Wan being romantically/relationally attached to someone, in the prequels, and in the Expanded Universe (EU), Luke had children with Mara Jade. Since a lot of the EU isn’t canon anymore, anything could happen.
What I do predict is that Rey will bring the prophesied balance to the Force, the balance that Anakin failed to bring in the prequels. But, one possible path Abrams, et al, could take is a Hegelian balance: she will end up being not a Jedi or a Sith, but a synthesis of the two opposing sides.
There was a hint of this Hegelian resolution theory that Mace Windu, one of my favorite Jedis, was the chosen one, and Windu’s death scene in Revenge of the Sith can support this. At that point, Anakin was still a Jedi, and Palpatine was a Sith. Notice how Windu’s willingness to kill a defenseless (seeming) Palpatine would’ve been an action both the Jedi and the Sith would oppose. Windu embodied the balance of the Force by synthesizing both Jedi and Sith, while transcending them both. Windu, both literally and ideologically, was caught between the two, and Anankin’s choice to side against Windu tipped the scales over to the Sith.
Abrams can emphasize this by making Rey a child of a Jedi—a Skywalker or Kenobi—and a Sith. Since Rey looks fully human, the mother would have to be human, which lends a little more credence to the Mara Jade scenario, if Abrams decided to rewrite her as a Sith.
If Rae will be the balance, this brings in another question: the Jedi (Luke and anyone else), and the Sith (Kylo Ren, Snoke, and anyone else), will have to somehow relinquish their use of the Force. If neither side is willing to do that, would Rey need to defeat them both…which means, if Luke is her father, she’d have to defeat him. This would tie up the prophecy plot line from the prequels, as well as end the literal cycle of the Jedi vs. Sith power struggle.