This is the only theory about Rey I'm actually OK with. I don't like any of the "Rey = X's daughter" theories, because their very philosophy makes things too "neat" by acting like everyone of importance must be related to a Legacy character (a.k.a. Finn Calrissian, which is just...really?). Plus, we already have a prominent descendant of the OT characters in Kylo, who is plenty. Rey being no one of importance, in reality rather than in appearance, aligns with the idea of Star Wars, which is that ordinary people can harness extraordinary power when necessary to maintain the balance of the Force.
Now, if JJ has decided to make Rey a second incarnation by the Force, I think I'd be OK with that, because it does offer up some interesting possibilities. If it is done, I'd like it to be done this way:
We know that the new canon has reintroduced the idea that Anakin was the product of a Sith meddling with the Force. In the EU, it was Darth Plagueis who "fathered" him, but the new, canon Vader comics seem to indicate that this has been switched to Darth Sidious himself, perhaps through knowledge he learned from Plagueis, who was known to experiment with the Force in order to "create life."
I think it would be interesting if Rey is also a product of someone meddling with the Force, more specifically the Light Side of the Force. Who do I think it should be? Kylo. Ben Solo was around 10 years old when Rey was born, so this seems odd, but it seems likely that he was beginning to explore his Force sensitivity by that time. Luke tells Rey in TLJ that Kylo was the first time he had ever seen someone with as much raw strength as her, so for a 10 year-old boy, such power would obviously allow him to do things--even inadvertently--that he would not understand. My "theory" (a.k.a. idea) is that Ben Solo accidentally created Rey through the Force while messing around with his newfound powers, causing her to be conceived as the daughter of a junk trader on Jakku, who was presumably married to another such trader. This gives Rey's "parents" additional impetus to sell her--not only do they need drinking money, she is an unwanted child conceived through an apparent miracle that would probably disturb them both greatly. We see in Rey's vision that she was abandoned when she was young (maybe 4 or 5). My guess is that her "parents" finally chose to be rid of her after first encountering her Force sensitivity, which further freaked them out.
So, in this "theory," Kylo Ren is Rey's "father" through the Force, which explains their Force connection (and, in particular, why it seems to last beyond Snoke's death), as well as her strength; it also preserves both JJ's desire to have a dramatic twist for Rey and Rian Johnson's decision to make her a nobody.