I recently read a theory about Rey. It was titled 'Why Rey is Obi-Wan's Granddaughter'. The writer made some pretty good points, but after reading it I was still convinced she was Luke's daughter. Then I started thinking, why would they want to make a movie about what happens in between Star Wars Episodes 3 and 4. And then it hit me. The reason they are making 'Rogue One' is because they want Obi-Wan to get married and have a daughter, so the daughter will grow up and marry Luke and also have a daughter, which they name Rey. Luke starts training Rey in the ways of the force, along with the other younglings. Kylo Ren then kills all the younglings, but Luke ends up saving Rey, and erasing her memory. He then leaves her on the planet Jakku, with the junk dealer 'Unkar Plutt'. She becomes a scavenger, and that's where 'Star Wars: The Force Awakens' story begins. Now, keep in mind, this is just a theory and most likely not true, but it would be pretty cool if Rey was related to both Luke and Obi-Wan.
Tell me what you think about this theory in the comments so I can continue on.