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.
One of the most dramatic moments could be when Ahsoka first realizes she's facing Vader, knowing he was once Anakin. It's a moment full of shock and disbelief. Another could be when they clash lightsabers for the first time, the sparks flying as the power of the Force is felt around them.
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 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 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.
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.