A plot could involve Darth Vader having a moment of hesitation when facing Princess Leia. He sees in her a strength and spirit that reminds him of his past self. This moment of doubt could lead to a series of events where he starts to secretly help her in small ways without the Emperor knowing.
One idea could be that Darth Vader discovers his connection to Leia earlier and secretly tries to protect her from the Emperor's more sinister plans. He might use his position within the Empire to subtly divert threats away from her.
One idea could be a story where Darth Vader discovers Leia's true identity earlier and struggles with his loyalty to the Empire and his newfound knowledge of his daughter. He could secretly protect her from the Emperor's wrath.
A plot could involve Darth Vader having visions of a different future where he and Princess Leia are on the same side. These visions start to haunt him, and he begins to subtly protect her from the Emperor's wrath. In the meantime, Leia senses a change in Vader's behavior and tries to figure out if she can use this to the advantage of the Rebel Alliance. For example, she might try to convince him to turn against the Empire completely.
A fan fiction could also explore a parallel universe where Anakin and Darth Vader are two separate entities that co - exist. They are forced to fight not because of Anakin's fall, but due to some external force that wants to see the two most powerful Sith - like beings battle it out. This could lead to an exploration of their different fighting styles and the power dynamics between them.
Another way to develop their relationship is through a shared secret. Maybe they both know something about the Force that the rest of the galaxy doesn't. This secret brings them together in a way that defies their initial roles as Rebel and Sith. They have to communicate and collaborate to keep this secret, and in the process, their relationship evolves from pure hatred to something more nuanced, like a begrudging alliance with a hint of mutual curiosity.
Perhaps they have to team up to convince a group of war - torn planets to join the Rebel Alliance. They use their unique abilities, Luke's connection to the Force and Leia's diplomatic skills, to sway the planets. Along the way, they also learn more about their family's history and how it can help in their mission.
Well, there's a fanfiction where Darth Vader finds Luke and Leia in a Jedi temple. The temple has some sort of ancient power or secret that they are all after. Vader initially wants to use the power for the Empire but then realizes that his children's connection to the Force and the temple is something he needs to consider. There could also be a fanfiction where Vader finds them in a diplomatic setting, like a meeting between the Rebellion and the Empire. This would create a tense situation where Vader has to balance his role as a Sith Lord and his new awareness of his family.
One idea could be exploring Darth Vader's identity before he became encased in the suit. Maybe he rediscovers his old self and tries to reconnect with his past relationships. Another could be him having to face the Jedi without the intimidating suit, relying solely on his raw Force abilities.
One plot idea could be a time - travel scenario where Ahsoka is sent back in time and has to face Darth Vader before he fully became the Sith Lord she knew. She could try to prevent his fall. Another idea is a story set in an alternate universe where Ahsoka and Vader team up against a greater threat, forcing them to put aside their differences. Maybe a plot where Ahsoka is captured by Vader and he has an internal struggle about what to do with her, given their past relationship.
Perhaps Darth Vader's dark side powers are misinterpreted in the Naruto world as a new and extremely dangerous form of jutsu. Naruto and his friends might try to find a way to seal these powers. Meanwhile, Vader is on a quest to find a way back to his own universe, and in the process, he starts to question his own dark ways after seeing the camaraderie among the Naruto characters. He could end up having a change of heart and use his powers for good in the Naruto universe.