In an alternate universe, Darth Vader could be exiled from the Empire after a disagreement with the Emperor. He then stumbles upon Leia on a remote planet. At first, they are enemies, but as they are forced to survive together against various threats on the planet, they start to understand each other. Leia realizes that Vader is not all bad, and Vader sees in Leia a determination that he once had. This understanding could lead to a slow transformation of their relationship from hostility to friendship, and eventually, they could work together to try and bring about a new order in the galaxy, one that is not dominated by the Empire or the Rebellion but a more balanced one.
In some fanfiction, their relationship might develop due to shared goals. Let's say they both want to stop a new superweapon being developed by the Empire. Working together towards this common objective forces them to interact more, and they begin to see each other in a new light. Vader might show moments of vulnerability, which Leia starts to pick up on, and she realizes there is more to him than just the menacing Sith Lord. This mutual discovery helps in building a more nuanced relationship between them.
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.
In some fanfiction, there could be a scenario where Leia discovers her connection to Darth Vader much earlier. Maybe she has a vision or some sort of Force - related encounter that reveals the truth. This would lead to a very different dynamic in the story as she grapples with this knowledge and how it affects her fight against the Empire. For example, she might try to reach out to the good in him, creating an internal struggle for Vader between his loyalty to the Emperor and his newfound connection to his daughter.
In fanfiction, their relationship could develop more quickly towards mutual understanding. Maybe Luke could convince Vader to turn away from the dark side much earlier, and they could start training together as Jedi. This would be a huge shift from the original story.
In some fanfictions, their relationship is very antagonistic. Darth Vader sees Ahsoka as a reminder of his failed Jedi past and wants to destroy her. Ahsoka, on the other hand, is determined to stop Vader's evil deeds.
In fanfiction, the relationship between Darth Vader and Padme is often a complex and tragic one. Many writers explore how Anakin's fall to the dark side affected their relationship. Some fanfictions might depict Padme's attempts to save Anakin from the dark side, while others may show how their love was doomed from the start due to the political situation in the Star Wars universe.
There might be some out there that are not overly adult - themed. However, in the official Star Wars lore, their relationship is one of enmity. Fanfiction that doesn't cross into inappropriate territory could explore different aspects like an alternate universe where they have to work together against a greater threat, perhaps with Leia trying to understand Vader's fall to the dark side in a more empathetic way.
Their relationship could start off as adversarial. Darth Vader sees R2D2 as just another piece of Rebel technology to be destroyed. But as the story progresses, he might start to respect R2D2's tenacity and the fact that it has survived so many battles. For example, if R2D2 helps Vader in some small way, like leading him out of a dangerous situation, Vader might start to view the droid differently.
In many fanfictions, their relationship develops in a complex way. At first, it's mainly about the enmity. But as the story progresses, there could be moments of understanding. For example, Vader might see some of his old self in Ahsoka. Ahsoka, on the other hand, might start to pity the broken man that Anakin has become. This could lead to a strange sort of respect between them, even though they are on opposite sides.
There's a fanfic titled 'Vader's Redemption'. In this story, Vader starts to question his loyalty to the Emperor much sooner, especially after some profound encounters with Luke. It's a great read as it shows the internal struggle of Vader while also exploring the bond between father and son in a new light. It's full of action-packed scenes and deep emotional moments.