I’m pretty sure Rey is just a force sensitive kid, and Finn is actually the one with significant parentage: Lando. Which would disprove your totally legit theory
I like and dislike the crash-land on Earth theory. On the one hand it would be very interesting and unexpected to many, but on the other, I kinda like to leave Star Wars in its own universe that never mingles with ours. But honestly, an Earth reveal at the end of Episode 9 or 12 seems to be quite plausible. Earth has been referenced in EU (Jedi Outcast being the most prominent example perhaps) and completely non-canon things. If Rey or Luke or both crashed on a planet that they don’t let us see, and then they get out and they see the Grand Canyon or Niagra, it would be pretty exciting.
In canon right now, Plagueis’s race isn’t known, so they could just change that. Also I heard a theory that he could be inhabiting different bodies like the plague that his name suggests.