A possible plot twist could be that Obi - Wan was actually faking being turned to the dark side all along. He was trying to infiltrate the Sith ranks to learn their secrets and stop them from within. He lets Anakin think he has succeeded in turning him, but in reality, he is still a Jedi at heart and has a plan to take down the Sith.
Perhaps the one who is really being manipulated is Anakin. There could be a hidden force, like an ancient Jedi spirit or a secret organization, that is using Anakin to turn Obi - Wan to the dark side for their own purposes. And when Anakin finally realizes this, he has to decide whether to continue or try to reverse what he has done.
Another plot twist could be that when Obi - Wan starts to turn to the dark side, he discovers that the dark side has a different origin than what the Jedi and Sith believe. This new knowledge makes him question everything he thought he knew. Instead of fully embracing the dark side, he tries to find a middle ground, which changes the dynamic between him and Anakin and the whole fanfiction's direction.
A plot twist could be that they are joined by other Jedi who also believe that the Dark Side has been misjudged. Together, they form a new order that tries to balance the Light and Dark. But this new order is seen as a threat by both the Jedi and the Sith. So Anakin and Obi - Wan have to fight on two fronts, against those who once were their allies and against the true Sith Lords who want to use them for their own evil purposes.
Perhaps Anakin manipulates Obi - Wan's emotions towards him. Anakin could play on their brotherly bond and twist it. He might make Obi - Wan think that by joining the dark side, they can save more lives together. Anakin could also introduce Obi - Wan to some dark side teachings that seem appealing at first glance.
Maybe Anakin uses a Sith artifact that has the power to influence minds. He manages to expose Obi - Wan to it without his knowledge. The artifact starts to corrupt Obi - Wan's thoughts, making him more aggressive and less Jedi - like. Anakin then further fuels this change by constantly challenging Obi - Wan's beliefs and actions, slowly pushing him towards the dark side.
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.
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.
In this kind of fanfiction scenario, it would be a huge shock to the Star Wars universe. Obi - Wan is known for his strong connection to the light side. If he turned, perhaps his relationship with Anakin would have been drastically different. Instead of being a mentor, he might have been a rival or even a corrupting influence on Anakin.
Well, if Obi Wan turned to the dark side in fanfiction, it would likely shake up the dynamics between him and the other Jedi. It might also change the outcome of many key battles and events in the Star Wars universe.
Some fanfictions might explore a situation where Anakin is being manipulated by a powerful dark force or a Sith Lord. This dark influence makes him see Obi - Wan as a threat to a grander plan. So, he kills Obi - Wan as part of this dark scheme, not fully in control of his own actions. And this can lead to a lot of complex emotions and plot developments later in the story, like Anakin's further descent into darkness and how the other characters react to Obi - Wan's death.
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.
One common theme is the struggle between light and dark within Obi - Wan. It shows how he might battle his own inner demons. Another is the impact on his relationships, like with Anakin or the Jedi Order.