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.
Anakin would probably be a respected Jedi Master. He would be living in the Jedi Temple, training new padawans. His relationship with Obi - Wan would continue to be that of a mentor - mentee relationship, but also a deep friendship. He would be a loving father to Luke and Leia, teaching them the ways of the Force from a young age.
Well, if Anakin didn't turn to the dark side, the Clone Wars would probably have ended differently. Anakin was a key figure in many battles during the Clone Wars. His continued loyalty to the Jedi could have meant a more decisive victory for the Republic. The Sith plot to create the Empire through the Clone Wars would have been foiled. And the relationship between the Jedi and the clones might have evolved in a more positive way. Anakin could have used his influence to improve the treatment of the clones, and they could have become an even more integral part of the Republic's defense force rather than being turned against the Jedi as they were when Anakin turned.
In such a fanfiction, Anakin would likely remain a Jedi. He would continue to fight for the Republic and the Jedi Order. He might have a more positive influence on the Clone Wars, perhaps finding a way to end the conflict more peacefully. His relationship with Padmé would also be different, not marred by his fall to the dark side.
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.
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.