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.
One of the most dramatic encounters could be their lightsaber duel. The contrast between Ahsoka's acrobatic and precise style and Vader's brute force is intense. Their duel is not just about physical combat but also about the ideological clash between the Jedi and Sith. Ahsoka represents the hope and the Jedi way that Vader once knew and discarded. Another aspect could be the moments of silent confrontation, where their expressions and the Force around them create a palpable tension.
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.
One way to further explore their relationship is through the Force. Since they are both strong in the Force, they could sense each other's emotions and intentions from a distance. For example, Vader might feel Ahsoka's determination to stop him, and it could remind him of her as a young Padawan. Also, a story could be written where they communicate through the Force without actually meeting, sharing memories and feelings about their past together.
Their relationship could be shown as very conflicted. Darth Vader is deep in the Dark Side, but Ahsoka still remembers the good in Anakin. She might be angry at what he has become, while also feeling a sense of pity. Vader, on the other hand, might see her as a reminder of his failure as a Jedi.
In a fanfic, it's really up to the writer. However, considering their abilities, Vader is extremely powerful with the Force, but Ahsoka is very agile and has unique fighting techniques. If Ahsoka can use her speed and agility to her advantage, she might be able to outmaneuver Vader. But Vader's brute strength and experience in combat are also not to be underestimated.
A really exciting moment could be when Ahsoka and Vader engage in a battle of wits as well as a physical battle. For example, Ahsoka might try to appeal to the Anakin still inside Vader, which could cause a moment of hesitation. Or Vader could taunt Ahsoka with memories of her past failures. The build - up to the final blow, whether it's Ahsoka's strike or Vader's, with all the emotions and power on display, would also be an extremely exciting moment in a fanfic.
In a fanfiction, Darth Vader might finally face his past with Ahsoka. Maybe he would have moments of hesitation in their battles, as memories of their time as master and apprentice resurface. This could lead to an internal struggle within Vader, torn between the Dark Side and his former self.
Some fanfics explore a more nuanced relationship. They could be on opposite sides, but they share a history. Ahsoka's presence in Vader's life serves as a reminder of what he has lost. Vader, in turn, is a symbol of the fall of the Jedi order to Ahsoka. Their relationship develops as they are forced to confront each other multiple times, each time learning more about the other's motives and beliefs.
Well, a cool plot might be that Ahsoka gets trapped in a Sith - created illusion world by Vader. In this world, she has to face her past traumas and fears related to Anakin's fall. Meanwhile, Vader is conflicted as he watches her struggle, memories of his past with Ahsoka resurfacing. Or there could be a story where they both are on a mission to retrieve a powerful Jedi/Sith artifact, and their paths cross in a race against time.
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.