Star Wars: The Last Jedi - 8 Parents J.J. Most Likely Wanted For Rey

What kind of Abrams list would this be if it didn't have a mystery box-style ending?

This theory was floated out on the Internet several times and at first glance, it may seem like a bit of a long-shot but Abrams may have come way closer to committing this than one may think. As has already been covered, Abrams love 'twists', so having Rey be the daughter and/or creation of a bad guy would be right up his alley.

But he also loves the Empire. Though it has been fleshed out and explained in-depth since, the First Order is essentially just the Empire 2.0 in Force Awakens. They use all the same materials and ships. They have the same master plan, revolving around a planet-destroying battle station. There are even surrogate stand-ins for Vader, Palpatine, and Tarkin from the original trilogy in the form of Kylo, Snoke, and Hux, respectively.

Hell, one of the original working titles for Force Awakens was Shadow of the Empire. Abrams wanted the Empire's presence to loom large over the film and what better way to do that than have Rey be a creation of Palpatine's own?

In the duel on Starkiller Base, Rey uses several different moves that were used by Palpatine in the prequel trilogy. Which could easily just be coincidence, (there are only so many moves one can do with a laser sword) or it could be Abrams' sneaky way of telegraphing such an impending twist.