I kinda hope that the importance of Jedha in the Force plays into Vader’s involvement. As in, Krenic battles it out with the Rebels for the first 2/3rds of the film, suffers a setback and retaliates by using the DS to destroy Jedha. Vader and the Emperor, both Force users become outraged and Vader is dispatched to deal with Krenic’s failures. Krenic walks into the room we see above, only to find Vader standing there. He tells him that the Emperor is ‘most displeased’ and gives Krenic, one last chance.

Krenic fails again, either on the DS or on Scarif, Vader kills him and systematically wipes out our happy band of Rebels only to have Erso survive long enough to get the technical readouts to one of the Organas (Bail or Leia)

Or something totally different