In such a fanfiction, one possibility is that Anakin Skywalker might initially be in shock upon seeing Darth Vader. He could struggle to come to terms with what he has become. Maybe there would be a battle of wills as Anakin tries to fight against the dark side within Vader.
Well, Anakin Skywalker's journey to becoming Darth Vader was a complex one. He was a slave on Tatooine as a child, and his strong Force abilities were noticed early. He was taken under the wing of the Jedi. But his mother's death, his love for Padmé, and the political intrigue in the galaxy all played a part. The Jedi's distrust of him and Palpatine's insidious influence pushed him over the edge. He believed that the dark side could give him the power to prevent the loss of those he loved, so he embraced it, thus becoming Darth Vader.
Anakin Skywalker's character development is a long and complex journey in 'The Rise of Darth Vader: A Star Wars Story'. In the beginning, he was a young, talented Jedi with a big heart. But his pride also grew as his power increased. He didn't like being told what to do by the Jedi Council. His relationship with Padmé was against the Jedi Code, which added to his internal conflict. As the story progresses, he faced more challenges and temptations. His desperation to save Padmé from a supposed death made him trust Palpatine more. Palpatine exploited Anakin's weaknesses, his love, his pride, and his doubts about the Jedi. This continuous manipulation and Anakin's own internal battles led to his downfall and the rise of Darth Vader.
Luke Skywalker could be another key character. Since he is Vader's son, his presence could add an interesting dynamic. He might be the one who somehow triggers the battle between Anakin and Vader, or he could be in a position to influence the outcome of their fight, perhaps by appealing to the remaining good in Anakin or by facing off against Vader himself in a three - way struggle.
It really depends on how you write it. If it's Anakin at his peak before turning to the dark side, he has a lot of raw power and potential. But Vader has years of experience as a Sith Lord. So, it could go either way.
Darth Revan might see Anakin as a powerful tool. He could try to manipulate Anakin further towards the dark side, using his own experiences of falling to the dark side to tempt Anakin. Maybe he would offer Anakin power beyond his wildest dreams if he fully embraced the dark side.
One idea could be a story where Luke discovers a way to temporarily reverse Vader's Sith corruption, leading to some heart - to - heart conversations between them as Anakin. Another could be a fanfic where they have to team up against a new, greater threat in the galaxy that neither the Rebels nor the Empire can handle alone. Maybe a story where Luke gets trapped in a Sith - created alternate dimension and Vader has to go in and rescue him to atone for his sins.
One common theme is redemption. Many fanfics explore the idea of Darth Vader still having a glimmer of Anakin inside and the possibility of his return to the light side. Another theme is the struggle with the dark side. Writers often depict Anakin's internal conflict as he fully embraces the power of Darth Vader. There's also the theme of lost love, especially his relationship with Padmé and how it affects him as Darth Vader.
One idea could be a what - if scenario where Anakin in his pre - Vader state somehow gets a glimpse of his future as Vader. He could then fight against the dark side within himself that would turn him into Vader. Another could be a time - travel plot where Vader is sent back to face Anakin, and Anakin has to decide whether to accept his possible future or fight against it.