I think Rey is Luke's daughter, or at least one of the children he was forming. Here are my clues (some more relevant than others):
->She is a skilled pilot living on a desert planet. Guess who was a freakin good pilot too ? Anakin and Luke.
->Kylo and Rey both have a long cloth draped around them, respectively black and light/white. Nobody else has it.
->The lightsaber calls to her and no one else, even though Kylo tells her it belongs to him.
->The vision/flashback shows Luke and R2, the Knights of Ren in the middle of the massacre from a subjective point of view, most likely hers. Why are the Knights of Ren not attacking her ? If she is indeed Vader's grandchild, Snoke/Kylo would want her to join them.
->Luke would have left her on an isolated planet to protect/shield her from Kylo/Snoke.
->When Finn asks how she freed herself on the base: "I don't know how I did it and you wouldn't believe me." She uses the force on instinct and already has some knowledge about it. ( resisting the mind intrusion, catching the lightsaber instead of Kylo, then beating him)
Why doesn't she remember all of this ? Either Luke erased her memory, or she was so traumatised by the abandon she turned it into a myth/forgot about it completely.
And my final point which particularly stroke me when I saw the movie again.
-> When Kylo Ren goes into her mind, he says roughly : "You're so lonely. At night desperate to sleep, you imagine an ocean... an island... Han Solo." Earlier in the forest, he already saw that she was getting attached to Han like a father figure. She projects her father on a island surrounded by an ocean.
And where does Luke hide ? An a frigging island on a ocean planet !!!
That was my two cents. Thank you for reading it all.
I'd love to discuss it