In some fanfictions, there could be a love interest that lures Luke to the Dark Side. Suppose a Dark Side user uses a romantic relationship to influence him. They could make him see the Dark Side as a path that would allow them to be together without the restrictions of the Jedi code. Luke, blinded by love, then turns, and his actions start to mirror those of the Sith.
Maybe in the fanfiction, Luke witnesses something so devastating that he loses faith in the Jedi way. For example, if the Jedi Order fails to prevent a great catastrophe. This could push him towards the Dark Side as he seeks a more radical solution to bring order. He might start using aggressive and unethical means, like Vader did, believing it's for the greater good.
One possible scenario could be that Luke is lured by the power promised by a powerful dark side figure. Maybe he's faced with a situation where he believes the dark side can save someone he loves, like if Leia was in mortal danger and the only way he thought he could save her was through dark side powers.
In some fanfictions, Luke might fall to the dark side after being betrayed by his closest friends. For example, if he discovers that they have been hiding important information from him that he thinks could have saved lives. This sense of betrayal could push him towards the dark side as he feels abandoned by the light. Also, a powerful dark side user could use mind control techniques on Luke, slowly turning him over time.
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.
If Luke Skywalker turns to the Dark Side in fan fiction, one possibility is that he could become a powerful and feared Sith Lord. He might use his Jedi skills in a perverted way, like using the Force to dominate rather than protect. His relationship with his friends would be severely strained. Leia, for example, would be heartbroken and might try to bring him back, but it would be a very difficult task. Han Solo would also be in a tough spot, having to choose between his loyalty to Luke and his own sense of right and wrong.
His friends like Han Solo and Leia would be deeply affected. They would either try to save him or be forced to fight against him. This would create a great internal conflict within the group that once fought together against the Empire. Also, Luke's turn could inspire other young Force - sensitive individuals to follow the Dark Side, thinking that he has found a better path.
One interesting 'Luke Skywalker dark side' fanfiction could be where Luke turns to the dark side after being constantly betrayed by the New Republic. In this story, he might use his newfound dark side powers to take revenge on those who he feels have wronged him. He could form an alliance with remnants of the Empire and become a powerful and feared Sith - like figure in the galaxy.
If Luke turns to the Dark Side in fan fiction, the story would take a very different turn. The relationship dynamics would be completely altered. His friends like Chewbacca, who had fought beside him, would be in shock. His fall could also inspire other Jedi - in - training to question their own paths. Maybe some would be tempted to follow him into the Dark Side. And the Emperor, if still alive, would have a new tool to use against the remaining Jedi and the Rebellion. He could use Luke to spread fear and chaos, which would be a very different plot from the original Star Wars stories.
One possible storyline could be that they face challenges from the dark side while Mara is pregnant. Maybe some remnants of the Empire try to target them because they see the unborn child as a potential threat or a powerful force for the light side. They would then have to go into hiding and rely on their friends and allies to keep them safe.
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.
Luke's friends from the light side could also be important. Characters like Leia, Han Solo, or Chewbacca. Their reactions to Luke's turn and their attempts to bring him back would be significant aspects of the story. There could also be new characters created specifically for this novel, perhaps some Jedi who were thought to be dead but are actually in hiding and oppose Luke's new path.