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.
The theme of redemption could also be present. Even if Luke is on the dark side in this novel, there might be elements of his past self trying to break through and find a way back to the light. And perhaps the theme of isolation as he may become alienated from his friends and allies due to his newfound alignment with the dark.
Power dynamics is a key theme. The dark side often represents a different way of using and abusing power compared to the light side. Luke's exploration of the dark side in the novel would show how power can be used for personal gain rather than for the greater good, and how it can change relationships and the overall balance in the Star Wars universe.
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.
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.
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.
It could be the remaining Jedi. Since Luke has joined the dark side, the Jedi who still uphold the light side would be his natural antagonists. They would see him as a threat to all they stand for and would try to stop him from spreading the influence of the dark side. For example, Leia, if she still adheres to the Jedi teachings, would be in direct opposition to Luke.
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.
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.
Sure. There's a fanfiction where Luke discovers a hidden Sith temple that has the power to reverse the light - dark side balance in a person. He enters out of curiosity but gets trapped. The temple starts to slowly turn him to the dark side. As he changes, his friends sense it and have to decide whether to try to save him or destroy him, which is a really unique take on the concept.
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 common theme is the struggle between the light and dark within Luke. Even as he turns dark, there are often remnants of his former self trying to break through. Another theme is his relationship with the other characters, especially how they view him now that he has fallen. For example, his friends might be in denial at first.