3) Retconning Star Wars
A cinematic galaxy as broad and diverse as Star Wars allows for a little reconfiguring of elements. As with the possible connections to Rebels, Edwards and his screenwriters (John Knoll, Chris Weitz and Gary Whitta) have some liberty to play with the established conventions. This wouldn’t be the first time new characters or technology was introduced after the fact, but it would be the first time after George Lucas’ departure.
For example, before Return of the Jedi, we’d never got much of a look at the Emperor and his entourage. As a result, the ominous, crimson-clad Imperial Guards were an added bonus. Similarly, before Jedi, the films hadn’t visited a wooded planet yet. Scout Troopers then entered into the franchise as a specialized element which the environment (and budget) allowed.
[zergpaid]Even though Rogue One takes leads directly to the formative period in franchise history, the universe is large enough to allow new elements. So yes, there may be another Dark Side force user (possibly one of the Knights of Ren as depicted in the trailer), but he or she won’t necessarily be a Sith Lord. And yes, there could easily be elite squads of what are being called “Deathtroopers” that operate beyond the scope explored in the Original Trilogy. That’s the magic of the Star Wars universe.
And the perks of meddling with an era just prior to A New Hope is that it opens the door for more exciting surprises hidden behind the vagaries of history.