Star Wars 9 Theory: Emperor Palpatine Created Rey (As An Anakin 'Clone')
2. Why Rey And Not Kylo Ren?

If Emperor Palpatine was still around and looking to create another Anakin Skywalker, then why wouldn't he simply use the person who believes he is the heir to Darth Vader, Ben Solo/Kylo Ren? There are two possibilities here.
One is that Kylo Ren was always the original plan. Luke, at one point, believed him to be the new Chosen One, before he was seduced to the Dark Side by Supreme Leader Snoke. Snoke himself says he was supposed to be a new Vader. If Palpatine was indeed pulling the strings all along, then it's not beyond reason he had Snoke seduce Ben because of that potential, but created Rey as a contingency plan. If Kylo fulfills his promise, then great. If not, then he's got Rey, with all of her UNLIMITED POWAH.
However, it could also be because of Darth Vader's 'failure', in that he turned back to the Dark Side, that Palpatine created Rey. That he needed someone not of that lineage to fully see his plan through, because Kylo would go the same way as his grandfather. But also, Episode IX is the end of the Skywalker Saga, which likely means that - redeemed or not - Kylo needs to die, which Rey doesn't.