1. Jyn is Rey's mom
After the Rogue One trailer dropped, millions of voices cried out with this fan theory and were silenced. I fear something terrible happened: internet haters shut down a perfectly good fan theory.
So, what's the evidence? Well, they both have British accents, and Rey's British accent was a vital piece of evidence in fan theories about her ancestry. Namely, it was the strongest (read: only) piece of evidence for Obi-Wan Kenobi being her father...or grandfather or uncle or ambiguous male relative.
Why would both Rey and Jyn have British accents? Again, if your argument is that Disney is allowing British actors to use their accents in the film, may I remind you that John Boyega is a Brit and he speaks with an American accent in The Force Awakens.
Let's put aside the vocal similarity. The two have a striking physical resemblance. Pale brunettes with big eyes. Because apparently that is the only way women look in the Star Wars universe.
The similarities don't end there. The two women fight in a similar way, whether it's shooting at storm troopers or smacking enemies over the head.
But Jyn's told old, you say? What are you trying to say, you age-ist? Old people can't have babies???...Okay, actually, the science is on your side there.
But hold up: Rey was born roughly 15 years after Rogue One.
In the trailer, Mon Mothma notes that Jyn has been "on [her] own since the age of 15." This sounds like the kind of thing an authority figure says to an intransigent youth.
Also, Mark Hamill was 26 years old when he played Tatooine Teen Beat Cover Boy Luke. Felicity Jones is 32, but she could easily pass for younger. This means Jyn could have had Rey in her late thirties, maybe early forties. That is not inconceivable. Pun not intended, unlike Jyn's pregnancy, which was definitely very intended and resulted in Rey. You heard it here first, folks.
2. Jyn becomes Captain Phasma
Another theory, one that some fans feel does not contradict the "J + L = R" theory, is that Jyn is Captain Phasma. The inspiration for this theory comes from Forest Whitaker's haunting lines in the Rogue One trailer. This trailer, much more than The Force Awakens's trailer, seems concerned with the protagonist turning to the dark side. There's even this haunting shot of Jyn, with the Empire's symbol on her arm, standing nearly consumed by darkness, before lights click on behind her.
We see this while Whitaker's strained voice -- was he being force-choked for the whole monologue? -- asks: "What will you do when they catch you? What will you do if they break you? What will you become?" His questions seem to foreshadow a transformation that Jyn's character will undergo during the film. Perhaps the answer to these questions is: Captain Phasma.
Like Darth Vader in A New Hope, Phasma wasn't unmasked in The Force Awakens so we don't know what hides beneath her helmet.
Yes, Jyn Erso and Captain Phasma are played by different actresses, Captain Obvious. (Good work! You've been promoted to Major Obvious!) So were Darth Vader and Anakin Skywalker. Perhaps Jyn turns to the Dark Side after a traumatic injury and, like everybody's favorite grandpa, Darth Vader, she got a new cybernetic body that's taller and tougher.
Everyone agreed that The Force Awakens mirrored A New Hope. George Lucas intended for the trilogies to "rhyme". Yes, RHYME "like poetry." (I don't think George has ever read poetry) These parallels are intentional. If the next two installments mirror Empire Strikes Back and Return of the Jedi, then the question of who Rey's parents are will be resolved by a character already present in the trilogy.
I mean, imagine HOW rad it would be if while fighting Rey, Captain Phasma said, "No, I am your mother."
3. Snoke appears in Rogue One
Redditor Snakie113 suggests another connection between The Force Awakens and Rogue One. As was explained in the Star Wars visual dictionary, Snoke has been around since the Clone Wars (you can read more on that here). This detail led some fans to theorize that Snoke is actually Darth Plagueis, a character from the Star Wars EU or Star Wars Legends.
via Star Wars Net
Legend has it that Darth Plagueis was the master of Darth Sidious aka Emperor Palpatine. Darth Sidious described to Anakin Skywalker how Plagueis used his powers to achieve immortality before Sidious turned on him and killed him.
Why would Snakie113 suggest that Disney would bring this Legend lore into the movies when they got rid of the EU faster than Han shot Greedo? Because of this one image from the Rogue One trailer:
A figure in a black hood and cape stands before a mysterious, carefully guarded column. Although we are pretty sure Darth Vader will make an appearance in Rogue One, this hooded figure doesn't resemble Vader. It looks more like Palpatine. Here he is in his signature black robe:
And that column looks an awful lot like a Bacta-tank, doesn't it? In the Star Wars universe, a Bacta-tank is used to treat injuries and suspend a person in healing. Remember when they put Luke Skywalker in one at the beginning of The Empire Strikes Back after his run-in with the Wampa?
When you look at the two glowing bacta-tanks next to one another, you can see that they are almost identical:
So, why would Emperor Palpatine be visiting what is clearly a carefully-guarded Bacta-tank?
Snakie113 submits two equally good explanations: The first is that Sidious wants to keep Snoke's body as a trophy. The second is that he wants to extract from Plagueis/Snoke the secret to immortality.
How does this lead to Snoke's return to power in Episode VII? We're not sure. Perhaps Rogue One will reveal how this happened.
Or maybe Rey's true mother is in the Bacta-tank: Jar Jar Binks.
4. Rogue One's cast are the Knights of Ren
Going with the theory that Rogue One will focus on good people turning to the dark side, let's take a look at another theory linking Episode VII and this spin-off. In The Force Awakens, we get a quick glimpse at the Knights of Ren in Rey's force memories.
They look like nothing more than a bunch of goths in homemade armor, right? Kylo Ren was a member of the Knights of Ren. This group's significance isn't explained in the movie. What we know about the Knights come from accompanying Star Wars books and press interviews. The Knights of Ren were a small faction of the Dark Side led by Supreme Leader Snoke.
The concept art, released in the book The Art of Star Wars: The Force Awakens, better clarifies how the Knights differ from one another:
Remind you of anyone? Take a look at promo pictures for Rogue One:
You might begin to see similarities between members of the Rogue One cast and members of the Knights of Ren.
For example, look at the Sniper and Diego Luna's character. They are both holding their guns in the exact same way.
And The Armory and Riz Ahmed's character. They both are wearing goggles and carrying many small items on their chest.
And The Heavy and Jiang Wen's character. They both have a specific type of armor that covers only one shoulder.
The Monk and Donnie Yen. Although their outfits don't look very similar, sample pages from the upcoming book, Rogue One: A Star Wars Story, describe Donnie Yen's character as a "spiritual warrior." This suggests that Donnie Yen's character could very well become The Monk.
The remaining Knight is called "the Rogue." Could this be Jyn Erso? I mean, she IS the main character of Rogue One.
This would explain Forest Whitaker's lines. They foreshadow this group's being captured and turned to the dark side.
Redditor brnin8 shared this theory on r/fantheories. At first, I was skeptical but as I read on, I felt myself being turned to his side. Impressive. Very impressive.