This is a theory I haven’t yet seen anyone else bring up, and it’s just crazy enough to work…
What about Midichlorians? If a virgin Shmi Skywalker can become spontaneously pregnant with Anakain through Midichlorians – i.e. parenthogenesis – why can’t someone else? Perhaps Rey’s mother went through the same thing and was terrified by it, leading her to abandon the child.
Right now, Kylo Ren is around 30 years old and Rey is about 20. What if, at the time Ben was taken away from his parents by Snoke at age 10, the Force decided someone was needed to help Ben and sought out a new parent to form another special child like Anakin? And instead of letting her potentially become corrupted by improper training and conflicting influences – either by Jedi or Sith – she managed to grow up on Jakku, innocent of the Force. It could possibly teach her internally, instictually. And she’s grown up to be tough and resourceful but also kind and caring, and she values all life (including droids). She’s a nearly perfect counter to the Dark Side and Snoke’s influence on Ben.
Or, my more interesting theory… what if there was no mother at all? What if it’s a Midichlorian child borne from Luke? Not mpreg style, but maybe just using his genetic material as a base and not impregnating anyone. Perhaps Luke is meant to fulfull a destiny the Force hoped Shmi would through Anakin.
And, what if the Midichlorians that created Anakin essentially are a “soul” of sorts? They would not so much imgregnate his mother as actually BECOME the child she bore, and therefore BE Anakin, until he died and released them. What if, the same Midichlorians that were Anakin sought Luke to recreate themselves into a new Anakin-strength person: Rey? She would eventually find Ben, the child that went astray, when she was old enough to help. This would make Rey literally Anakin reincarnated, and therefore Kylo Ren isn’t “praying” to his grandfather for help, he’s praying to Rey. The light saber would naturally gravitate directly to her, and Kylo would have a reason to say “it is you” with such wonder. And Maz would have a reason to say she sees the same eyes in new people, and keep the light saber until “Anakin” returned for it.
What if the Force is trying to undo the damage it did with Anakin by sending the same batch of Midchlorians to create Rey? To make a Yin to the Yang, and possibly defeat the Dark Side for good.