Forgive me if somebody already stated this, but, I think the opening of Rogue One A Star Wars Story should open much like how Superman II opened, with flashbacks of what came before. They can show the beginning of the Empire from Revenge of the Sith until just before this story begins.
Have flashbacks of the Emperor and the Senate, Vader wiping out the remaining Jedi and alien worlds, more building of the space station known as the “Death Star.” with the John Williams Imperial March theme in the background.
That’s how I would do it. It’s different from the crawl.
They can also incorporate Mon Mothma scenes from ROTS into this as well.