Here is what I think:

Rey is the sister of General Hux.

To have any kind of impactful reveal to the origins of Rey, she has to be related to atleast one of the characters we have already seen. If it is just some random offscreen character then there is almost no point even of showing her origin.

This leaves us with just a few choices:
1. Luke
2. Han and Leia
3. Finn
4. Snoke
5. Poe
6. Hux

Luke seems like the obvious choice, but I don't think that is correct. a) This will feel too cliched to the point where there is no impact to the reveal. b) Also if that was so, their would have been some mention of Luke's wife in Ep 7. If she is alive, then there would be talk of where is she right now. If she is dead, there would be talk of how Luke went to live by himself due to the sorrow of her death. The fact that none of that is mentioned makes it highly probable that Luke never married.

Han and Leia are out since they would have recognized their daughter if they saw her. Not to mention how little urgency they showed in trying to get her rescued compared to that shown by Finn and how Han barely showed any concern for Rey running off on her own when Kylo landed at Takodana.

Finn is black, Rey is white. Dont mean to be racist, but reality... Not to mention Finn's origins are fairly clearly established and he seems like a clear love interest for Rey. No surprises here.

I dont think Snoke is related to anyone.

That leaves us with just Poe & General Hux.

Hux & Rey being brother and sister plays similar dark vs light side duality like Vader and Luke, while not completely ripping off the exact same formula. This is also the reason why I think Poe is not related to Rey.

There has been a lot of criticism around Hux being a young man, when an older person would seem to better fit the title. However him being brother to Rey explains this very well. He is a young general on the dark side and she is a young powerful Jedi . Both highly accomplished siblings. One joins the dark side due to his childhood atrocities, other prefers to remain a nomad than be evil. Both rise to power and eventually battle each other.

Hux being another major dark side villain, also turns the story in a direction where Kylo is not the soul focus. Thus it makes the new trilogy have a much more different story than the originals while still keeping similar tones.

Note that Hux is played by Domhnall Gleeson, who is already an accomplished actor. Unlike Rey, Finn, and Kylo who are played by actors for whom this is really their first lead movie role. If Hux had a minor role to play, not only would he be played by a no name actor with a lot less screen time, but would have been killed off at the end of Ep 7, just like his counterpart Governor Tarkin was killed off in Ep 4.

He is also the only character on the dark side to get the most screen time after Kylo Ren. Especially compared to Captain Phasma who is shown a ton in trailers but barely gets any screen time. I believe the emphasis on Phasma rather than Hux in trailers is just another ploy to get you not to think about Hux as much and thus having the eventual reveal be even more of a shock.

Since the structure of the movies is similar to the original, I believe the reveal will happen at the end of Ep 8, just like Ep 5. I also believe there will be a final battle in Ep 9 between Kylo and Rey. Kylo will be winning, but Hux attacks Kylo, saving Rey and sacrificing himself. This is similar to how Ep 9 ended with Vader attacking the Emperor and saving Luke.