I think something similar to the slow version of the “Force Theme” that was featured in the Rogue One trailer - https://www.youtube.com/watch?v=Glb5zqPdKSQ
should be used, because it fits the darker, modern tone that ‘Rogue One’ has.
Personally, I was fine with simply “A long time ago…” in the film. But I do look forward to see how the crawl can be integrated while matching the tone of the film.