SPOILER ALERT: THIS IS ALL MY OWN THEORY... but...
In one of the new cannon books, Palpatine has an underground base on Jakku #fact. If you watch TLJ with the idea in mind that Rey is a genetic experiment created from Luke's missing hand on Bespin, it's EVERYWHERE. The movie has closeup after closeup of Rey's right hand in key moments. Enough so that they'll look like easter eggs (to enjoy after the big reveal in Episode IX).
The very first moment in TLJ trailer was a clue. I think the entire mirror scene IS her realizing her worst fear, that she IS a clone and has no family. Her "parents" that sold her off for drinking money were indeed just that... scavengers that found Luke's hand and sold it to the Empire.
I'm just as psycho about Star Wars as all of you, and I remember the very first draft leaks and rumored outline of the The Force Awakens (at that time) started with Luke's hand floating through space after the opening crawl. TFA's first draft was going to be about Finn & Rey finding Luke's hand... I just think they switched it up and realized that could be a big reveal. My 2 cents, I think Rey also has Palpatine genes in her DNA milkshake. Because she always fights with anger (among other reasons).
This also "solves the problem" of her being too powerful, too soon. Kylo Ren clearly knows about "A GIRL" in TFA and is upset. He seems to go straight into trying to turn her let her know "she needs a teacher" because he has an idea of who she is and where she comes from. I haven't watched TFA for a long time but I would wonder if her hand is as called out in there as well...