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 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.
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.
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.
Palpatine could still be the main antagonist even if Luke turns to the dark side. He might try to use Luke as a more powerful weapon against any remaining Jedi or the Rebel Alliance. Palpatine would see Luke's newfound dark side power as a means to further his own control over the galaxy.
In some fanfictions, 'dark side Luke' might be paired with Palpatine. Palpatine, as a master manipulator, could have a role in Luke's fall to the dark side, and their partnership would be full of political intrigue and dark Force - related activities. Palpatine could use Luke as a tool to further his own evil plans, while Luke might be seeking more power from Palpatine.
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.
The Emperor or some other powerful Sith lords would likely be involved. They would see Luke as a great prize to turn to the dark side and would use all their cunning and power to manipulate him. Yoda's spirit could also be present, trying to guide Luke from beyond the grave but perhaps being unable to prevent his fall. Han Solo might also play a role, either trying to rescue Luke or being a victim of Luke's new dark side - influenced actions.
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.