The obvious answer
We'll find out how the Rebellion got their hands on the Death Star plans. Well, duh. But does that alone really shed any new light on the events or characters of the main saga? Face value, not really, no. We already know that the Rebellion managed to somehow steal the plans, and that's all we need to know to move the story along; how they were stolen is mostly irrelevant. Now, that's not to say that this movie is irrelevant, on the contrary, I think it's, for starters, going to be an important portrayal of the Rebellion's "first victory over the evil Galactic Empire." In addition, based on everyone that's involved and what we know so far, it has the potential to be a really good movie, even for people not necessarily interested in Star Wars (i.e. crazy people). So, then, are you saying this whole post is just click bait and you don't think any saga secrets will be revealed? Well, maybe...just kidding, no, I don't think the how of the plot is going to give us any inside information, but the who, on the other hand, may be where we hit the jackpot...
Who do you turn out to be?
That's the real question with the potential to unlock what saga secrets might exist in Rogue One; who are the characters going forward, and how might they interact with our beloved main protagonists? Is one of them perhaps the enigmatic Lar San Tekka? Or if not him specifically, a fellow member of the Church of the Force that might reference San Tekka as someone who could potentially help the Rebellion...maybe by suggesting they enlist the services of an old hermit on Tatooine...maybe we find out Princess Leia was headed there with the intention of telling Obi-wan he was their only hope in person before being sidetracked by her over-protective father, er, I mean, by the evil Lord Vader...that would be an interesting way to give San Tekka a little backstory association with Leia and the Rebellion so he doesn't seemingly come out of nowhere (which is exactly how he feels to me right now). Or, in a different direction, maybe one of them is an ex-Kanjiklubber...okay, fine, that doesn't really do anything for the saga, but it'd be cool as hell nonetheless...anyway, what if a somewhat popular theory rings true and the character Felicity Jones plays ends up being an important love interest for a certain Skywalker?
Do you take this rebel to be your wife?
For the record, I don't see this happening. I imagine if there are any nuggets of knowledge to be gained pertaining to the Skywalker story that they're more like my Lor San Tekka example and not monumental game changers like introducing Luke's future wife. That said, though, it's maybe not extremely farfetched; especially if you take into account that Felicity Jones has an accent similar to that of Daisy Ridley's, and, if you're in camp #ReySkywalker (with all the cool kids, like me), that would go a long way to explaining how a lowly scavenger from Jakku ended up speaking the Emperor's Basic (like the King's English, get it?)...keeping in mind that I believe Luke is Rey's father, I've been assuming that the accent must come from her mother, and, if Jones's character keeps her accent for the film, I'll at least consider the option that she's the mother of the galaxy's next new hope...if her character sounds like a bland, middle-American newscaster, though, well, that theory goes completely belly-up in my book.
What about the Ghost crew?
What about them? Yeah, I know, some people actually think Felicity Jones is playing Sabine, but that's just crazy talk...as Pablo Hidalgo explains in the Rebels Recon episode for "Wings of the Master," the Phoenix Squadron rebels and the group that ultimately destroy the first Death Star aren't on the same team yet:
So I guess the short answer to the title question is "probably none." But, fear not my friends, there still exists the possibility of saga secret beans being spilled in Rogue One, and, even if none are, it's still sure to be a not-to-be-missed, action-packed Star Wars adventure, the likes of which we've not seen on the silver screen...and that alone is worth the price of admission and the long wait outside the theater for the first showing. Maybe when the convention season gets into full swing and more details about the film get released I'll revisit this topic and see what I can add, subtract, multiply or divide based on the information given, but, until then, keep pondering the unknowable and start really getting excited about the fact that there is so much more Star Wars coming our way than I imagine anyone ever thought possible...and, of course, while you're doing it, may the Force be with you...