"Rogue One" is the first of many "Star Wars Anthology" films. Unlike "The Force Awakens" and its sequels, which will directly continue the original saga, the Anthology films aim to expand the larger Star Wars universe and focus on other characters and conflicts. For example, the next Anthology movie (due May 2018) will focus on a young Han Solo.
The Anthology films will also shake things up in terms of style and presentation. Don't expect all the familiar Star Wars tropes to be on display here. For one thing, John Williams didn't compose the score (that responsibility instead fell to Michael Giacchino), and we've learned that "Rogue One" won't even feature the iconic Star Wars title crawl.
For another, this movie will break from tradition by featuring flashbacks to Jyn's childhood.
Basically, don't expect a 100% traditional "Star Wars movie" this time. But with all the complaints about "The Force Awakens" rehashing familiar tropes, maybe that's a good thing.