He might be shocked by the advanced technology. The Empire's weaponry would seem primitive compared to future tech. He could also be intrigued by the new forms of the Force if they had evolved.
Darth Vader could meet advanced droids that have personalities and abilities far beyond what he's used to. These droids might see him as a relic from a bygone era. He could also meet future scientists who are intrigued by his cybernetic enhancements and try to study him. Additionally, there's a chance he meets descendants of his former allies or enemies, which would create a complex web of relationships and interactions.
Enemies could be many. In the past, he might face Sith Lords who don't want him to change the timeline and disrupt their plans. In the future, he could encounter new galactic powers or beings with strange powers that oppose his presence. For example, a future race that has developed technology to counter the Force.
Darth Vader would likely be conflicted. On one hand, he could see the potential in his younger self to become a powerful Sith. On the other hand, he might be reminded of his own lost innocence. He might give his younger self some warnings about the path he is about to take, but also encourage the dark journey.
If Darth Vader travels to a time when he was still Anakin Skywalker, he might have a chance to redeem himself earlier. Seeing his younger self could make him realize the mistakes he made and try to change the path that led to his dark transformation. His character could become more complex as he battles with his past self's idealism and his current self's dark nature.
In a fanfiction scenario, Darth Vader time traveling to the Clone Wars could also lead to some interesting battles. He might bring with him advanced technology or fighting techniques from the future. This could make him a formidable force on the battlefield. He could face off against some of the great Jedi of that era in new and unexpected ways, changing the dynamics of combat and potentially leading to different outcomes for various battles.
Their relationship could become more understanding. As they travel through time and see different events together, Luke would gain a deeper understanding of what led his father to become Darth Vader. Vader, in turn, might see Luke's unwavering hope and goodness more clearly, which could soften his stance.
Time travel could offer Darth Vader a chance to relive key moments and make different choices. For example, he could go back to the moment before his fall to the dark side and resist Palpatine's temptations. This would completely change the course of the Star Wars saga. Maybe he would see the error of his ways earlier and not cause so much destruction.
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.