A cool idea could be a fanfiction where Vader has a vision of an alternate future where he stayed with Padmé and turned away from the dark side. This vision haunts him during his missions in the Star Wars Rebels era, making him more introspective and less ruthless for a while. He might start to make different decisions based on this vision, much to the surprise of the Emperor and his subordinates.
There could be a fanfiction where Vader has an unexpected alliance with a rebel. Maybe they are forced to work together to stop a greater threat, like a rogue Sith or a powerful alien race that wants to destroy both the Empire and the Rebellion. This would put Vader in a complex situation as he has to deal with his loyalty to the Emperor and his newfound, albeit temporary, partnership with the rebel.
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.
In a fanfiction battle between Ahsoka and Vader, it's really hard to say who would win. Ahsoka is highly skilled and has a deep connection to the Force. But Vader is extremely powerful with his Sith abilities. It could go either way depending on how the writer wants to portray it in the fanfiction.
Ezra Bridger would be a great character to pair with Darth Vader. Their interactions could be really intense, as Ezra is a powerful young Jedi in the Rebels series and Vader has a history with the Jedi. It could lead to some great battles and also some interesting moments of understanding or misunderstanding between them.
One interesting story could be about Darth Vader secretly having a change of heart during the events of Star Wars Rebels. Maybe he has a brief encounter with a Rebel that makes him question his loyalty to the Empire.
One interesting fanfiction could be about Darth Vader secretly having a moment of hesitation when facing a Rebel who reminds him of his past self. Maybe he lets the Rebel escape without the Emperor knowing. It adds a new layer of complexity to his character.
Kanan Jarrus is another good choice. As a Jedi, he has the skills to face Vader, at least to some extent. Their interactions could explore the differences between the Jedi way and the Sith way. And Kanan's relationship with his crew could also play into the story, as he tries to protect them from Vader's wrath.
In some fanfiction, there could be a plot where Ahsoka and Vader's battles are not just physical but also mental. They engage in a battle of wills across the Force, with Ahsoka trying to break through Vader's dark side conditioning and Vader trying to corrupt Ahsoka. This creates a very tense and complex dynamic between them.
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.