In the fanfiction, Luke could be influenced by his family ties to the dark side. If he discovers some hidden connection to Vader or Palpatine that he didn't know before, it might make him question his place on the light side. He could think that joining the dark side is a way to understand his true heritage better and gain more power to protect those he loves in a different way than the Jedi would allow.
One possible plot could be that Luke is lured by the power the dark side offers. Maybe he gets frustrated with the slow progress of the light side in bringing order to the galaxy. He might see the Sith as more efficient in getting things done. For example, if the Republic is still facing numerous threats and the Jedi are tied by their code not to take extreme measures, Luke could be tempted to cross over.
Well, one possibility is that she gets frustrated with the slow progress of the 'good side'. Maybe she sees all the bureaucracy and inefficiency in the fight against the dark forces and decides that the dark side can offer a more direct way to achieve her goals, like a more efficient magical society. But this would be a huge shift from her original character.
In this fanfiction, it could be that Ezra is deceived by a powerful dark side figure. He might start to believe that the Jedi way has failed him and that the dark side can provide a more effective means to achieve his goals. This could lead him to make choices that gradually draw him deeper into the dark side, like using dark side powers and distancing himself from his former Jedi allies.
One possibility is that he is lured by the power promised by the dark side. Maybe he gets frustrated with the slow progress of the light side in fighting the Empire and thinks the dark side can offer a quicker solution to end the tyranny. He could then start using dark side powers like Force lightning and become a feared figure, turning against his former friends.
It could be the remaining Jedi. Since Luke is turning to the dark side, the Jedi who still uphold the light side values would be against him. They would see him as a threat to the balance they are trying to maintain and would try to stop him from fully embracing the dark side.
One possible plot could be that Luke is disillusioned with the Rebellion after some major setbacks. Maybe he discovers some secrets about the Rebellion that make him question their motives. So, he decides to join the Empire in the hope of bringing about change from within. He might start off by using his Jedi skills to gain favor among the Imperial ranks.
If Luke joins the Dark Side, it would completely change the dynamic of the Star Wars universe. His friends would be shocked and heartbroken. Leia, for instance, would probably try to bring him back. The Rebellion would lose a great hope and the Empire might gain an unexpected and powerful ally. It could also lead to a different kind of struggle within the galaxy as those who once believed in Luke now have to face him as an enemy.
Another plot could involve a powerful Sith Lord who manipulates Luke. The Sith Lord could use Luke's insecurities, like his fear of not being strong enough to protect his friends. He could be tricked into believing that the Dark Side would give him the power he needs, and slowly turn to the dark side as a result.
Well, perhaps Harry gets tired of being constantly in danger and feeling unappreciated in the wizarding world. He might be lured by the Dark Side with promises of power and safety. Voldemort could offer him a place by his side, showing him a different view of magic that seems more appealing in a warped way.
In fanfiction scenarios where Naruto turns to the dark side, it could start with a personal tragedy. For example, if Hinata was killed by someone from the Leaf Village under false accusations. This could shatter Naruto's belief in the village and its values. He would then be vulnerable to the influence of the dark side. Once on the dark side, he could use his charisma to recruit others who have also felt wronged by the ninja world. He might form his own group, with different rules and goals from the traditional ninja villages. His journey to the dark side would not only change him but also send shockwaves through the entire ninja world, as he was once seen as a symbol of hope and a force for good.