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.
Luke Skywalker could be another key character. Since he is Vader's son, his presence could add an interesting dynamic. He might be the one who somehow triggers the battle between Anakin and Vader, or he could be in a position to influence the outcome of their fight, perhaps by appealing to the remaining good in Anakin or by facing off against Vader himself in a three - way struggle.
Well, Obi - Wan Kenobi has shown time and again his ability to adapt to different opponents. He defeated General Grievous, a formidable foe. Pong Krell, on the other hand, was consumed by his own dark desires. When it comes to a fight between them, Obi - Wan's use of the Force for defense and offense, his lightsaber skills which are refined over years of training, and his wisdom in combat situations would make him the victor. He can sense his opponent's moves and counter them effectively, something Krell's overconfidence might not be able to handle.
In such a fanfic, the Avengers' diverse powers would clash with Darth Vader's formidable Force abilities. Iron Man could use his high - tech suits to try and outmaneuver Vader's lightsaber attacks. Captain America might lead the charge with his shield against Vader's dark side - enhanced combat skills. It would be an epic battle of good from different universes against Vader's Sith - influenced evil.
It really depends on how you write it. If it's Anakin at his peak before turning to the dark side, he has a lot of raw power and potential. But Vader has years of experience as a Sith Lord. So, it could go either way.
In the fanfiction where Darth Vader travels back in time, his bond with Obi - Wan Kenobi would be full of possibilities. Imagine Vader going back to a time when he was still Anakin. He might have a newfound respect for Obi - Wan, knowing all that Obi - Wan tried to do for him in the future. Obi - Wan, for his part, could be shocked to see this changed Anakin. They could form an alliance against the emerging threat of the Sith, or Vader could try to get Obi - Wan to understand his view of the Force. Their relationship would no longer be simply that of a Jedi and his fallen apprentice, but something far more nuanced and unpredictable.
In fanfiction, their duel could be more about psychological warfare. Ahsoka might use her knowledge of Anakin to try and get into Vader's head, making him question his loyalty to the dark side. For example, she could bring up memories of their time as master and apprentice. This would add a layer of complexity not always seen in the official duels.
She changes it a great deal. For starters, the gender difference can bring in a new type of relationship. Instead of the more traditional male - male rivalry, there could be an element of attraction or different kind of respect. Also, her way of handling the Force and Jedi teachings might be different, which would affect how Vader reacts to her.
One interesting idea could be a female Obi - Wan being a mentor to a younger generation in a different way than the male Obi - Wan. She might use her unique perspective to train Jedi. As for Vader, a fanfiction could explore his relationship with this female Obi - Wan in a more complex way, like they have a grudging respect for each other's skills.
One possible plot could be that Vader, without his suit, is found by Obi - Wan in a hidden location. Obi - Wan, despite their past, decides to help Vader recover. As they interact, they start to understand each other's perspectives better. Maybe they even form an uneasy alliance against a new common enemy in the galaxy.