"Is Rey Descended From Emperor Palpatine?
First things first: This theory runs with the common belief that Rey's father is Luke Skywalker. I previously wrote about my conviction that she is actually a Kenobi, but as plausible as that is, it does seem that the sequel trilogy is so completely centered around Luke that Rey being his daughter is kind of inevitable.
Clearly, we don't know what happened to Luke in the years between the death of Vader and his recruitment of students to train at the Jedi Academy on Yavin 4, but it's within the realm of probability that he met somebody and procreated. This is Star Wars: Everyone has kids all over the joint.
Now to the question of Rey's mother: What if she was a descendant of Emperor Palpatine? Count Dooku never revealed that the Emperor was in fact Darth Sidious, so there's no way of knowing whether or not the people of the New Republic ever became aware of his double identity, but let's imagine that, at least among some factions, word got out.
Crucially, Luke knew, and a personal connection with Palpatine is probably not something his wife or lover would want to become public knowledge. Something happened to frighten her — perhaps the secret of her bloodline was discovered, and a third party used the knowledge as blackmail material — causing Rey's mother to run away with the child. In theory, this could have happened before Rey's birth, before Luke even knew he had a daughter.
If the above sounds totally far-fetched, it's worth bearing in mind that Luke's wife in the Thrawn trilogy was Mara Jade, also known as the Hand of the Emperor — and although that has been struck from canon, Grand Admiral Thrawn's re-entry into official canon could signal the introduction of a character who parallels Mara Jade into the new trilogy, if not Mara Jade herself."