I think Rey is Luke’s clone, obtained from his severed hand that was left in Bespin.
The empire recovered both hand and lightsaber and created her as a failsafe, but lost her to Luke and his allies. That’s why she has the mechanical and piloting adeptness of Anakin without the lesser traits of Solo. She’s still technically Kylo’s cousin, but by far the better Vader than he can ever dream of. This is the relevance of the recovered lightsaber: it showed that interested parties has been to the cloud city to recover valuable artifacts.
Luke realizes this when Rey was a little girl: it was dangerous to train her as a Jedi, she is also too dangerous to be trained under Snoke. Short of killing her, Luke just leaves her in the isolated world of Jakku.
So why is Rey a woman? Apart from the fact that a male clone will look exactly like Luke, I suppose the movie will explain that Snoke created both male and female variants, but only Rey showed true promise as a Force wielder.
Kylo will try to be killed by Snoke in favor of Rey, and will be forced to return to the light. Rey will succumb to her destiny. Except for Finn. Not only does he love her, he represents the theme of the third trilogy: you are not your past.
The last movie in the latest trilogy will be about Finn getting Rey back from the First Order.
Based on this theory, I therefore make the following predictions:
- Snoke will abandon Kylo Ren as Rey is the far superior Vader descendant.
- Kylo, after realizing neither the Jedi or the Knights of Ren are the answer to his dilemma, will choose his own path to the Force. At the end of the trilogy, he will set off as a lone wanderer.
- Kylo will first help bring down the First Order before leaving.
- Luke will be a force ghost by Episode IX, having spent the past decade training for it.
- Rey will be seduced to the Dark Side not only because she was created for that purpose, but also because she will discover that Luke abandoned her in Jakku in the first place.
- The other Knights of Ren are the failed male Luke clones, identical to him but possessing less force abilities.
- Rey’s genes has been mixed with Velociraptor, tree frog, and cuttlefish DNA.
It’s been revealed that Maz Kanata has force powers in a deleted scene. So this makes her a force-wielder who has not sided with the Jedi or the dark side.
Since I speculate that Kylo Ren will walk a path that is neither Jedi nor dark side, Maz can show him the third option.