Rogue One will mark the first time we get to see a Star Wars movie set apart from the main movies chronology. So, we as fans don't really know what to expect from this completely original story or what secrets it could hold.
After Star Wars: The Force Awakens ended, fans were left with far more questions than we could even hope to have answers for. It seems like an impossible task for all of those to be answered in the upcoming Episode VIII. Maybe Rogue One will go ahead and help take some of the pressure off director Rian Johnson to deliver on all the answers fans will be looking for.
Whether it full fledged answers or just simple hints and clues, I think these five mysteries could be solved after the release of Rogue One:
Obviously the Jyn Eroso is Rey's Mother drum has been beaten so hard that there's a hole in it, but it still remains a plausible theory. All the pieces are there for this to be proven as true, despite how some officials are waving off the rumor. I'd be denying it to if fans already guessed our big twist.
Whether it's Jyn or not, this film could still introduce Rey's lineage or provide some clues. If she isn't a Skywalker then maybe her parents will be traced back to the Rebellion is some fashion. I believe they're saving her big reveal for Episode VIII though, and I have my own theories that I may write on soon.
This is where things could get a little exciting, because they've seemed to skip over Finn lineage a bit and given no real hint or clue to go on. While he is still years from being conceived during the time of Rogue One, his parents may be running around with the Rebellion or even the Empire.
Finn said in The Force Awakens he was taken by the first order as a child, but gives no specifics as to why he was one of many children taken. Perhaps we will get a better look at how The Empire selects its Stormtrooper trainees in Rogue One and maybe at least be thrown an Easter egg.
Could we maybe even see the already fan favorite Saw Gerrera hold some key to Finn's origin? Maybe Saw get's captured in the movie and forced to work for the Empire and has a son forced to be trained by the First Order. Pretty big stretch considering the timeline wouldn't seem to match after the Empire was destroyed, but you get the point - anything's possible.
Sharon Duncan-Brewster's Senator Pamlo is another interesting idea for possibly Finn's Mother.
Captain Phasma's Origin
We were very disappointed with her minor role in The Force Awakens, but Rogue One could provide us a back story to the character. The way she was hyped up
Phasma is roughly 40 years old during The Force Awakens, which would make her around 10 during Rogue One. For being such a high ranking official in the First Order it seems likely that she would have had some affiliation with the Empire prior to the First Order. At the very least maybe her parents worked for the Empire.
Young actress Boriana Williams is in an undisclosed role and fits the age for Phasma.
The Beginnings of The Knights of Ren
If the Film Theorist is right, then this movie could end up being a Knights of Ren origin story. The way Snoke speaks of them in The Force Awakens would imply they're a long standing division.
If I could piggyback off of MatPat's theory, I think we could be looking at a re-imagining of the video game The Force Unleashed with Jyn Erso taking the place of Starkiller in the movie.
She comes to rescue her father but is captured and seduced by Vader to join him where he trains her in secrecy as the first Knight of Ren. It's a reach, but would be interesting if true. Would also explain why Ben Solo became so fascinated by them, because his Grandfather started them.
Also is it a coincidence that Jyn's fathers name is Galen just like Starkillers real name was in the game? Maybe a decent hint by the writers.
Supreme Leader Snoke's Identity
This guy is old and has to be doing something at the time of this movie, and if he created the first order I don't see how he didn't have his hands in the Empire.
We know literally nothing about him other than he is extremely ugly, so should we be looking for an atrocious alien with a bad facial deformity wearing an imperial uniform during the movie? It would make sense to possibly have a hint hidden somewhere in the film.
I am 99% sure Snoke's identity much like Rey's lineage will be save for a big reveal in Episode VIII, but don't be surprised if an Easter egg is thrown out there.