He had to be there ... In the Senate. So no when you are on set doing another film or show running production 16 hours a day you don't have any time to go onto another site & shoot 6 scenes anywhere else. Even a green screen, which half of his scenes could not have been done which makes your entire point moot.

Vader's appearance in Rogue One has been & will always be the worst thing they ever did in that movie. 30 seconds of fan service completely sold out the film & overshadowed the sacrifice of the heroes that WERE the STARS of the movie.

He stole the show & immediately you forget just what they did for. It's disgustingly gratuitous. I hate it. I understand why so many people like it & it's exactly what is wrong with so many things in the world today. Bad priorities.

If true then Krillin would be next. He trained with Master Roshi, Goku, & his wife could help. He has had the best training out of anyone other than the main characters (& actually WAS the #2 character for awhile). Gohan would be able to teach him to transform.