One common theme is redemption. Vader gets a chance to go back and correct his wrongs, which is a big draw for many readers. It gives a sense of hope that even a character as dark as Vader can find a path to the light.
There's also 'Time - Traveling Vader: A New Destiny'. Here, Vader goes back in time and teams up with his younger self. It explores the complex relationship between the two versions of him and how they can alter the course of the galaxy together. The story is full of action and some deep emotional moments.
Yoda could also be an important character if the time travel somehow involves the time when he was still alive. He could offer his wisdom to Luke and perhaps even try to reach out to Vader's remaining light side, given his powerful connection to the Force.
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.
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.
Another plotline could involve them traveling to the future. Maybe they see a world where the Jedi have been fully restored and are working in harmony with the galaxy. This could make Vader question his past actions even more, and Luke could gain new insights into the future of the Jedi Order.
One interesting plot could be Darth Vader traveling back in time to prevent his own fall to the dark side. He might try to change the events that led him to become Vader, like saving Padmé. Another could be Vader traveling to the future and seeing the downfall of the Sith, which makes him question his loyalty to the dark side. He could then try to find a way to change the future of the Sith order.
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.
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.