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.
One way a fanfiction could explore Anakin not turning to the dark side is by emphasizing his own inner strength and his connection to the light side of the Force from the very beginning. Maybe he has a different mentor within the Jedi Order, someone who really understands him and helps him deal with his emotions in a healthy way. This could prevent him from being vulnerable to Palpatine's temptations. Also, the fanfiction could show how the Jedi Order as a whole could change its ways, becoming more inclusive and less dogmatic, which would also make it easier for Anakin to stay true to the light side.
In fanfiction about the 'dark side of justice', authors often play with the concept of justice being misused or corrupted. It could be a hero who turns bad due to a flawed justice system. For example, in some stories, a vigilante might start off with good intentions but then go too far in their pursuit of justice, becoming more of a dictator in the name of justice.
One way could be to explore the idea of a great betrayal. Maybe a powerful Sith Lord manipulates events in such a way that Anakin and Obi - Wan both feel they have no other choice but to side with the Dark Side. For Anakin, it could be something related to his fear of losing Padmé, and for Obi - Wan, perhaps the Jedi Order's actions against him make him lose faith. The story could start with a series of small incidents that gradually push them towards the Dark Side.
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.
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.
Maybe the Force itself could be an antagonist in a sense. The light side of the Force would be trying to pull Obi - Wan back from the dark side. And the natural balance of the Force that doesn't want too much dark side influence could be working against Anakin's plan to turn Obi - Wan. So the Force, in its own way, could be the main antagonist.
It could be the remaining Jedi. Since Rey is turning, the Jedi who still uphold the light side would see her as a threat. They would try to stop her from causing more harm with her newfound dark side powers.
The fanfic could be set during the Clone Wars. Anakin's dark side starts to emerge due to the stress and manipulation. Padme notices the changes in him. Meanwhile, Luke and Leia are prophesied to be the ones who can save the galaxy from the dark side that Anakin is falling into. It could show how their fates are intertwined even before they are born, with Padme trying to protect the future of her children and Anakin.
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 possible aspect of the 'Jack Frost Dark Side' in fanfiction could be his struggle with his powers. He might be tempted to use them for self - gain or revenge. For example, in some fanfictions, he could be frustrated with how humans treat winter and decide to make it harsher than it should be, causing chaos and destruction.
In fanfiction, Sakura Haruno's dark side could be shown as her being more power - hungry. Maybe she's tired of being in the shadow of stronger characters and decides to take extreme measures to gain power. This could involve using forbidden jutsu or making alliances with villains, which is quite different from her usual kind - hearted self in the original series.