Palpatine, of course. His influence on Vader and their complex relationship can be delved into. In fanfiction, we could see more of their power struggles or how Vader might try to break free from Palpatine's control in ways not shown in the movies.
One interesting Emperor Vader fanfiction could be about his early days as Anakin Skywalker and how his fall to the dark side was different from the canon. Maybe in this story, he has more internal struggles that are explored in greater detail.
Darth Vader could encounter advanced droids that have developed personalities and even some form of Force - like abilities in this future. These droids might be a new challenge for him as they are not like the droids he is used to dealing with in his own time. He may also meet future versions of his former allies or enemies, who have been reincarnated or cloned in some strange new way, with different personalities but still with a connection to their past selves.
In the 'naruto is like darth vader fanfiction', their journey of self - discovery could be seen as similar. Naruto has to constantly figure out his place in the ninja world and how to control his power. Darth Vader is on a journey of self - discovery too, as he grapples with his identity as Anakin Skywalker and his role as Darth Vader. They also have a sense of isolation in some ways. Naruto often feels alone in his struggles, and Darth Vader is isolated in his dark armor, cut off from his former self and many human connections.