THEORY:
Abrams and Kasdan wrote The Force Awakens with no definitive answer as to who Rey's parents are, but did so in a such a way as to give Episode VIII writer/director Rian Johnson essentially three options to choose from. In other words, the only concrete "clues" as to Rey's heritage in TFA are rather scenes/exchanges/instances that can be retroactively interpreted to fit whatever path Johnson / Lucasfilm choose to take...
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FACT: The Force Awakens takes place "roughly 30 years" after Return of the Jedi.
According to The Force Awakens Visual Dictionary:
FACT: Kylo Ren is roughly 30 years old during the events of TFA.
FACT: Rey is 19 years old during the events of TFA.
FACT: In the immediate months / year after Return of the Jedi, Han and Leia give birth to Ben. Because of his uncle's and grandfather's immense power with the Force, in order to ensure that Ben's Force-sensitive nature isn't corrupted by the Dark Side, at a young age, they "enroll" him Luke's Jedi Academy / ensure Luke is there to help guide him toward the Light.
- OPTION 1A: 11 years later, Han and Leia give birth to a second child... Rey.
- OPTION 1B: 11 years later, Luke and an yet-to-be-identified partner give birth to a child... Rey.
- OPTION 1C: 11 years later, Rey is born to yet-be-identified parents; parents who are likely not part of the Skywalker bloodline.
FACT: Despite Han, Leia, and Luke's efforts, Ben is somehow still corrupted by Snoke / the Dark Side, likely due to Ben's obsession with his grandfather's legacy, and takes on the name Kylo Ren.
FACT: Rey is five years old when she is abandoned by her family and left orphaned on Jakku.
- OPTION 2A: At age 16(ish), Kylo Ren murders his fellow Jedi apprentices. Fearing for their daughter's life, who will no-doubt also be immensely Force-sensitive, and thus of great interest to Kylo/Snoke, Han and Leia hide/abandon their daughter on Jakku, just as Luke was hidden on Tatooine. These traumatic events ultimately lead to Han and Leia separating, each reverting back to their former lives.
- OPTION 2B: At age 16(ish), Kylo Ren murders his fellow Jedi apprentices. Fearing for his daughter's life, who will no-doubt also be immensely Force-sensitive, and thus of great interest to Kylo/Snoke, Luke hides/abandons his daughter on Jakku, just as he was hidden on Tatooine. This traumatic event ultimately leads Luke to go into hiding, blaming himself for not only having to abandon his daughter, just as he was abandoned so many years ago, but also blaming himself for not being able to suitably protect Kylo from the Dark Side.
- OPTION 2C: At a later age than 16, Kylo Ren murders his fellow Jedi apprentices. Rey's abandonment on Jakku is then unrelated, because it happened before Luke goes into hiding and/or Han and Leia need to hide their daughter from Kylo/Snoke
A few scenes/exchanges/instances in TFA that can be interpreted to fit the above criteria...
- When Han first encounters Rey and Finn on the Millennium Falcon, he was there (in that system, close to Jakku) smuggling, but also could have been there keeping an eye on his daughter. He hasn't seen her in 14 years, so A) he either doesn't recognize here, or B) pretends not to. Same goes for her being Luke's daughter in that scenario.
- Either way, under the Skywalker bloodline scenario, Han could likely figure out who Rey is pretty fast, and is forced to play coy. Then, when approaching Maz's castle, Rey makes a wide-eyed remark that she didn't know there was this much green in the whole galaxy. The camera then cuts to Han, giving a wry/knowing expression. This could be interpreted as either A) I feel guilty for abandoning my daughter on Jakku, B) If she only knew Luke was her father, or C) I've never seen this kid in my life, and it's pretty crazy that she's never seen trees before.
- Soon after, Maz asks Han, "Who's the girl?" and it then cuts away to Rey going after Finn, trying to convince Finn not to leave. Whatever Han told Maz, it then causes Maz to follow after Rey into the castle basement. Following Rey's flashback/vision, Maz then talks to Rey as if she knows A) Rey is Han and Leia's daughter, B) Rey is Luke's daughter, C) Rey is simply, but obviously, very strong in the Force.
- The one big hole in Rey-being-the-daughter-of-Han-and-Leia is Han and Leia's private conversation at the Resistance base, where they only discuss their son. That said, Han could potentially still not know that Rey is the daughter they abandoned, and thus no need to bring it up with Leia. In the meantime, Leia could then figure out that A) she's their daughter or B) she's Luke's daughter, and either way, thus why Leia seems to warm to Rey so much in the end.
- Either way, there are also the hints that Kylo seems to be extremely interested in "the girl," and not just because she helped Finn and BB-8 escape; something else that could be attributed to Rey either being the sister or cousin he may have been aware of.
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Ultimately, after learning Kylo and Rey's ages, and considering the fact that Luke was committed to training new Jedi - and thus still committed to being a Jedi - I just can't see him having a wife/partner, breaking the Jedi code, and having a daughter after restarting the Jedi Academy. That said, it's still just as hard for me to believe that Han and Leia would abandoned their daughter, then not discuss it at all when we see them privately. But as I've laid out above, I do think there's enough vagueness and room for interpretation to believably/retroactively have Rey be their daughter.
If I had to bet on it right now, it's actually more likely to me that Rey is either the daughter of Han and Leia, or potentially Obi-Wan Kenobi's granddaughter, rather than Luke's daughter. Not only did Ewan McGregor record a special line-reading of the name "Rey" for the flashback/vision sequence in TFA, but knowing he was basically the last Jedi in existence, save for Yoda, with no plans to start an Academy himself, I could see Obi-Wan ultimately breaking his vow on Tatooine, having a kid (likely one he didn't raise), and then that kid ultimately giving birth to Rey.
I still have no clue, but point is, I don't think Abrams and Kasdan did either, and Johnson seems to have been given the green light to go in whichever direction he wants...
