My two cents:
I think it's obvious Rey is a Skywalker because of how much she is like Anakin. A Mary Sue who can pilot well and fix anything. These are not traits of Han but of Luke/Vader...
I think she is Luke's daughter, or at least was trained by Luke as a small child before coming to Jakku. First of all, when the force "awakens", it seems like she knows what she's doing. She doesn't look like someone with newfound powers, but someone who is remembering old training.
Most importantly, when one of the officers tells Kylo about how BB8 escaped with a girl, Kylo exploded... almost as if he knows of a force sensitive girl that may be an issue...
My theory is that this girl, Rey, trained with Kylo under Luke before Kylo turned crazy and Luke sent her to Jakku to protect her. Kylo knew she was Luke's daughter, so he was afraid of her potential power, which both explains why Luke hid her from him and why Kylo lost his shit when he first heard about her appearance after all those years...
Not the most crazy theory out there, but this is star wars, not metal gear solid. It seems like the most logical explaination...
(edit) aaaaand zebra beat me to it lmfao...