This new Rogue One: A Star Wars Story rumor comes from makingstarwars, who has a great track as far as this series is concerned - so just a smidge of salt is needed. According to the website, the plot for the next installment in a galaxy far-far away takes its cues from the biggest and deadliest discovery of the 20th century: the atom bomb.
Gareth Edwards, director, has already described the movie a very realistic one, without any force users, with those involved in the war almost always having a dual agenda: “It’s the reality of war. Good guys are bad. Bad guys are good. It’s complicated, layered; a very rich scenario in which to set a movie.” (via Comic Book Movie)
So what will Rogue One be about? makingstarwars says that one of the key characters of the movie, the one which will set the plot rolling, is Galen (portrayed by Mads Mikkelsen). He is the scientist who took the faulty design of the Empire's Death Star and made it a foolproof one. Of course, we already know that the Death Star was easily destroyed - so Galen must have been an incompetent scientist.
But it isn't so - just like J. Robert Oppenheimer who made the Manhattan Project a successful one, Galen also realizes that the Empire will use the Death Star to bring destruction. So he defects and joins the rebels, which explains how the latter ones got their hands on the Death Star schematics.
In order to succeed in his plans, Galen co-opts his only daughter, a character portrayed by Felicity Jones. The introduction of this character is essential, since their turbulent relationship will be the one to bring the stakes of Rogue One: A Star Wars Story (the outcome of his defection is already known, with the Death Star being easily destroyed).
The website also reports that the dangers are very high, since not all of the members of the team will survive in the end. Also, makingstarwars reveals that the first trailer for Rogue One: A Star Wars Story will drop with the debut of Captain America: Civil War.
