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.
Try Wattpad. There are some really interesting Anakin recently Darth Vader fanfics on there. Some writers focus on the internal struggle of Anakin as he transitions to Darth Vader. They explore his emotions, his relationships with other characters like Padmé and Obi - Wan in this new context. Just search for 'Anakin Darth Vader' in the search bar and you'll get a list of relevant stories.
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.
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.
One idea could be a story that focuses on Anakin's internal struggle between his love for Padme and his fall to the dark side as Darth Vader, while Luke and Leia unknowingly grow up in the shadow of their father's actions. It could explore how their lives might have been different if Anakin had not turned, and how they eventually come to terms with his true identity.
A possible plot is a fanfic where Anakin, in his Darth Vader form, has a vision of a different future where he didn't lose Padme. This vision makes him question his loyalty to the dark side. Meanwhile, Luke and Leia are on a mission to rescue their father from the clutches of the Emperor. The story could be filled with emotional reunions and battles between the light and dark forces.
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.
The idea of redemption is a big one. Darth Vader's redemption at the end of Return of the Jedi is a powerful moment, and fanfics often expand on this. They might explore what led to his final choice, or how his family influenced his decision. Additionally, the exploration of the Force and how it connects all these characters is another common theme. It could be about how Luke and Leia develop their Force abilities differently, or how Anakin's relationship with the Force changed over time.
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.