One main factor could be the influence of a powerful dark side figure. Such a figure might manipulate Luke's emotions, like his fear or anger. Maybe they prey on his insecurities about his abilities or his place in the galaxy.
One possible fanfic could be about Anakin's slow descent into the dark side while his love for Padmé still lingers. And how Luke later discovers the truth about his father. Maybe in the fanfic, Anakin has internal struggles between his love for Padmé and the power the dark side offers, and this conflict could shape the events that lead to his transformation. Luke's discovery of his father's past could be a central part of the story, as he tries to come to terms with it and perhaps find a way to redeem his father's actions.
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.
Well, obviously Luke himself is a key character. He is the one who is falling to the dark side, so his actions, thoughts, and decisions are central to the story.
One of the main themes could be the struggle between good and evil. Luke Skywalker is typically a symbol of the light side, so exploring his journey towards the dark side would likely involve his internal battle with his own moral compass. Another theme might be the allure of power that the dark side offers and how it can corrupt even the most noble of characters like Luke.
One of the main themes is the struggle between good and evil within Luke himself. In these fanfictions, we see how his inner conflict plays out. Also, the theme of redemption might be present in some, where there is a possibility for Luke to come back from the dark side. And the exploration of the dark side's allure, like the promise of quick solutions to problems compared to the more difficult path of the Jedi.
One interesting 'dark side Luke' fanfiction could be about him leading a Sith cult. In this story, Luke uses his knowledge of the Force in a twisted way, recruiting followers and causing chaos across the galaxy.
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 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.
Perhaps a fanfic that delves into the time when Anakin was on the cusp of turning to the dark side. Padme could sense the change in him and tries to prevent it with the help of Luke and Leia from the future who somehow travel back in time. This would create a complex web of relationships and a race against time to save Anakin from the dark side.