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.
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.
Fanfiction usually develops Anakin's character by showing his relationships in a new light. His bond with Palpatine could be shown as more complex, with Palpatine slowly manipulating Anakin over a long period, preying on his weaknesses like his desire for power to save Padmé. This makes his turn seem more gradual and believable.
The fanfiction could explore an alternate timeline where Anakin has stronger support from the Jedi Council. If they recognized his potential and didn't hold him back, he would be more likely to stay on the light side. It could also show how his relationships, like with his mother or with Padmé, could be developed in a way that keeps him grounded in the light. For instance, if his mother was saved in a different way and he didn't have that anger and fear, it would help him resist the dark side.
Another plot is Anakin's search for greater power in fanfiction. He believes that the Jedi Order is holding him back from reaching his full potential, and the Dark Side seems to offer unlimited power. So he makes the fateful decision to join it. This often involves his internal struggle between the light and the dark within him, which is a great source of drama in these fanfictions.
If Anakin didn't turn to the dark side in fanfiction, it's conceivable that he would have become a true hero. He could have trained more Jedi, passed on his skills and wisdom, and maybe even prevented the rise of new villains. The story would be filled with hope and optimism instead of the darkness that followed his fall.
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.
Their relationship becomes more possessive. In the Dark Side, Anakin's love for Padme could turn into an obsessive need to control her. He might see her as his possession rather than an equal partner.
Some fanfictions explore Anakin's turn from the perspective of his friendship with Obi - Wan. Maybe there's a rift between them caused by outside forces, like a false accusation against Anakin that Obi - Wan doesn't fully believe him about. This slowly sours their relationship and Anakin, feeling abandoned by his friend, is more susceptible to the dark side's allure.
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.