The Force is definitely a key element. In the fanfiction, the way the Force is used by both Ahsoka and Anakin is important. Ahsoka might use the Force to reach out to Anakin in ways that other Jedi couldn't. Also, the setting can be an element. If the story is set during a time of great turmoil in the galaxy, like during the Clone Wars, the external pressures on Anakin would add to the complexity of his situation and Ahsoka's task to save him. It could also be that the fanfiction involves other characters who either help or hinder Ahsoka's efforts, like Palpatine, who might try to stop her from saving Anakin.
A key element would be the relationship between Ahsoka and Anakin. Their history, trust, and mutual understanding play a big role. Another element is the nature of the dark side's hold on Anakin. Understanding what led him to the edge of the dark side is crucial. For example, if it was his fear for Padmé's life, then Ahsoka would have to address that fear somehow.
In some fanfictions, Ahsoka might save Anakin through the power of their strong bond. She could remind him of their shared memories, the good times they had together, and the Jedi values they once held dear. By appealing to his emotions and the part of him that was still good, she could potentially pull him back from the dark side.
Well, at first, they might seem closer as they are now both part of the same 'dark' world. But soon, the dark side's influence of greed and power - hunger would start to drive a wedge between them. Anakin, with his already tumultuous relationship with power, might try to dominate Ahsoka. Ahsoka, having been dragged into the dark side against her will perhaps, would resent Anakin for it. Their relationship would be a volatile mix of shared darkness and individual power struggles.
One plot idea could be that Anakin, already on the dark side, tries to convince Ahsoka to join him. He shows her the power that the dark side can offer, like how quickly she could gain the ability to defeat her enemies. But Ahsoka is conflicted as she still has a strong bond with the light side teachings of the Jedi.
One aspect of Ahsoka in dark side fanfiction might be her change in appearance. She could start wearing darker robes and her lekku might even seem to have a more menacing look. Her fighting style would also likely change, becoming more aggressive and less about defense. Writers often explore how her relationships with other characters, like Anakin, would be affected. Anakin might be torn between trying to save her and seeing her as a threat.
If the dark side saves Harry in fanfiction, it could open up a whole new world of story possibilities. Maybe it would involve Harry having to confront his preconceived notions about good and evil and find a way to balance the powers within him.
One common theme is redemption. Even though Anakin has turned dark, there's often a glimmer of hope for him to return to the light through his connection with Ahsoka. Another theme is the struggle between loyalty and the dark side. Ahsoka has to decide how to deal with a dark Anakin, whether to try and save him or oppose him.
Betrayal is a common theme. For example, she might be betrayed by the Jedi Order, which pushes her towards the dark side. Another theme is power. The allure of greater power, whether it's political or related to the Force, can be a factor in her fall.
In fanfiction, Anakin joining the Dark Side often has complex motives. It could be due to his fear of losing Padmé. The Dark Side might seem to offer him a way to save her, so he succumbs to its power.
The exploration of their different personalities is also important. Anakin is complex, with his internal struggles, while Ahsoka is more level - headed at times. Fanfics might show how Ahsoka tries to understand Anakin's dark moods or how Anakin is proud of Ahsoka's achievements. For instance, when Ahsoka successfully completes a difficult mission, Anakin's reaction can be a great source of character development in the story.
Betrayal is also a big theme. He betrays the Jedi Order, his friends, and ultimately himself. His relationship with Obi - Wan, for example, is often explored in these fanfictions as a relationship that is shattered by his turn to the dark side. And there's the theme of internal conflict. Anakin is constantly at war with himself, his good side fighting against the allure of the dark side, which is a central aspect in many of these stories.