1. He is Luke's son.

Everyone was quick to name Rey as Luke's kid, but that's been all but disproven by The Last Jedi. Finn, though, has shown a few signs of force sensitivity: Snoke senses an awakening in the force after Finn breaks out of his brainwashing and before Rey finds Luke's lightsaber, he sensed/heard the attack on the Republic on an entirely different planet, he was able to handle a lightsaber not once, but twice in battle, a weapon only force sensitive people are capable of controlling, etc. So where did that force sensitivity come from? Could this be a bait-and-switch on the previous bait-and-switch that was advertising Finn as the new main character and then swapping him out for Rey? Not to mention the way he rescues Poe in The Force Awakens mirrors the way Luke rescues Leia in A New Hope AND stealing the son of Luke Skywalker away to place in your brainwashed reboot of Anakin Skywalker's army would definitely be a power move.

2. He is a lost prince.

This theory mainly stems from misinformed promo material after multiple sources printed that Finn was a prince from Artorias, a backstory that actually belongs to Finn Galfridian of the Star Wars extended universe. But, given that 1) the EU isn't technically canon anymore, and 2) the writers for the sequel trilogy have been pulling from the EU as inspiration, it's not too far-fetched to guess they might pull a little from the original Finn's storyline. After all, Finn Galfridian was also force sensitive and later trained with Luke to become a Jedi, both ideas I would happily watch play out on the big screen. Plus, imagine the leverage the First Order would have if one of the children they kidnapped was actually the prince of a powerful nation.

3. He is Lando's son.

This theory continues to feel a little like a cop-out to me since Lando and Finn are basically the only two black characters in the entire franchise, but Billy Dee Williams is returning as Lando for Episode IX, so I definitely wouldn't rule it out.

4. Naomi Ackie is playing his sister.

This isn't exactly about Finn's parents, but it is by extension. Naomi Ackie was recently cast in Episode IX and the theories have been flying ever since. The current popular guess is that she is playing "Caro," a character which casting calls describe as "an 'ethnic female', with a strong preference for an African-American, 18-26 years old. Caro is described as having a 'captivating naturalness and ease to her manner.' Caro is a 'leader and problem solver, smart with a great sense of humor and a strong will. All this comes to her effortlessly.'"

A few months after that casting call went out, Naomi Ackie was cast. She is, conveniently, around the same age as John Boyega. She is from the same area of London as John Boyega. She was spotted on set filming with John Boyega. The connections may be coincidence, but they also feel very, very deliberate.

Who their parents would be is a bit of a mystery, but Ackie was also photographed during filming concentrating intensely at a lot of nothing that Finn and Poe seem to be running away from, hopefully suggesting she'll be a force user at the very least and, by extension, that her parents (and perhaps Finn's) were as well. Fingers crossed.

5. All of the above.

Luke married Lando and their children, force users Caro and Finn, were meant to be raised as prince and princess of Cloud City and that's just how it is.