Rogue One: A Star Wars Story strikes me as being an example of fan fiction that just happens to have been produced by the owners of the intellectual property. I say this as the highest form of compliment.
Fan fiction started off with the Star Trek community, as authors (mostly female) distributed “slash” stories (like Kirk/Spock) detailing gay relationships between principal characters, and other salacious ideas that would never have been approved by producers or censors. But as fandom became more popular (and respectable), fanfic evolved to a more professional quality, and fans even got to making their own video productions, like James Cawley’s Phase II (creating new adventures for the original Star Trek characters years before J.J. Abrams’ 2009 film). But the main thing these productions had in common is that they were creating original stories for established characters (or an established setting) that the owners of the property didn’t want to produce themselves. But Paramount Pictures, the owners of Star Trek, seem to have reversed their tolerance for such things, quashing the recent fan project Star Trek: Axanar with a lawsuit.
Which from a fan perspective is too bad, because these ideas help expand the concept of what is possible in a fictional setting and ask questions not answered in official “canon.”
For example: What happened in the nearly 20 years between Star Wars Episodes III and IV?
The Star Wars prequels established that Palpatine had been planning to build his Death Star years before he became Emperor, and before Luke Skywalker was born. Rogue One is the story of how the new Rebel Alliance plotted to gain the plans to the space station, hoping to learn its structural flaws. (‘Spoiler alert- they found one.’ -Jimmy Kimmel) It centers on former Rebel Jyn Erso (Felicity Jones) who, like Rey in Episode VII, is a strong, likable heroine who is at the center of the action rather than being a support character or damsel in distress. She is recruited by Rebel intelligence officer Cassian Andor (Diego Luna), who wants her to find her father, an Imperial scientist, but doesn’t tell her exactly why. Their mission goes south but they learn that the Empire has just completed its “planet killer” space station, and when the Rebel Alliance refuses to organize, Jyn resolves to find the plans to the base herself. As such, the movie takes cues from those old World War II movies where commandos have to perform a secret mission in occupied Europe, and you know someone is getting killed, you just aren’t sure who and how.
This greater realism (relative to Star Wars) is increased by the fact that apart from Darth Vader (voiced by James Earl Jones!), there are no Jedi in the piece, although martial arts star Donnie Yen plays a variant of the Blind Master archetype, who was a monk at one of the last temples of the Force. This shift in emphasis is important in at least a minor way, given that while you did have a vast universe to explore with the Star Wars setting, the stories so far have mainly been about the journey of a prospective Jedi into mastery – while Luke (and Rey) had a large group around them with their own stories, once they developed their powers, they started spending more time away from the team. The prequels, meanwhile, were almost entirely about the Jedi Order.
So that in itself makes Rogue One, as launching point for Lucasfilm’s “anthology” concept, very valuable. It ISN’T really stand-alone, given that the story ends almost exactly at the point where Star Wars (Episode IV: A New Hope) begins. And again, we know how that worked out, and it isn’t too hard to guess what happens to these characters. But they are given a certain level of depth that the main series (especially the prequels) were not known for. Put another way, if you have an acquaintance who for some reason can’t stand Star Wars, you might ask them to see Rogue One with you. It works as a Star Wars story, and it works outside of being a Star Wars story. I hope it is a sign of things to come.