Honestly, before watching TFA I was on the "Rey is Luke's daughter" bandwagon. After seeing the film and reading the novelization that all changed.
The first big clue for me was the effect Anakin's saber had on Rey. How it showed her visions and came to her aid in the fight with Kylo. The saber had a supernatural connection to her.
Upon my second viewing, I paid more attention to her vision at Maz's. I was puzzled by the shot of her on Bespin. I kept asking my friends "What was the point of that?" I can understand someone that's Force-sensitive having visions of the future but not of the past, specially a past that doesn't belong to them. On my third viewing I noticed it wasn't random at all,
Then I read the novelization.
As I finished that paragraph, this immediately flashed in my head.
That's when it hit me. "THE FORCE AWAKENS". More like ANAKIN AWAKENS - inside Rey. 20 years in "Limbo" as punishment for 20 years of murder and rage come to an end the moment Rey finds the saber. Anakin's key memories and abilities at that point begin to slowly emerge (Palpatine, Bespin, Jedi Mind Control, Lightsaber training). He is awake now, inside her.
What intrigues me the most about the whole theory, is whether or not Luke knows that the girl handing him that saber is his Father.