Luke might turn to the Dark Side if he experiences a great personal loss. Suppose his friends or family are killed and he blames the Jedi way for not being able to protect them. In his grief and anger, he could be lured by the power that the Dark Side offers. It gives him a false sense of control and the ability to seek revenge. Also, if he were to be exposed to powerful and alluring dark side artifacts or knowledge, it could corrupt his mind and make him question the teachings of the Jedi.
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.
One reason could be that he's in deep meditation as a Jedi. Jedi often fast during their meditative periods to enhance their connection to the Force. So, in the fanfiction, this could be why he's not eating.
A fanfiction idea could be that Ezra, in his search for a way to defeat a great enemy, stumbles upon an ancient dark side temple. Inside, he is influenced by the dark side presence. His connection to the light side starts to fade. He becomes more aggressive and less empathetic, using his powers for his own gain rather than the greater good, and his relationships with his former allies become strained as a result.
One possible reason could be extreme loss. If Cal were to lose all his friends and allies in a tragic way, he might feel so much pain and anger that it drives him towards the dark side. For example, if his newfound Jedi family was wiped out by the Empire, the grief could cloud his judgment and make him vulnerable to the dark side's temptations.
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 in a fanfiction could be that Ezra is lured to the Dark Side by the power it seems to offer. Maybe he gets frustrated with the slow progress of the Rebellion and the constant threat from the Empire. He might be tempted by a powerful Sith Lord who shows him visions of a galaxy where he has the strength to end all the chaos, but at a great moral cost.
If Luke turned to the Dark Side, the Rebellion would be in a very difficult position. He was a symbol of hope and a powerful Jedi. Without him on their side, the morale of the Rebellion would drop significantly. The Empire, on the other hand, would gain a very powerful ally. Darth Vader might even try to make Luke his apprentice, and together they could become an almost unstoppable force.
It could be a new Sith Lord created specifically for the fanfiction. This new character could be more powerful and devious than any before. He could be the one who lures Luke to the Dark Side and then becomes his enemy as they compete for power within the Dark Side hierarchy.
One possible reason could be a misinterpretation of his actions. Maybe he made a decision that others in the fanfiction misconstrued as betrayal, like his attempts to reach out to the enemy in an effort to bring peace but it was seen as siding with them.
There could be a fanfic where an ancient Dark Side artifact is discovered near the Ghost. The crew decides to study it, not realizing the danger. As they do, the Dark Side energy starts to affect the ship itself. Systems malfunction, and strange things start to happen. Kanan, with his Jedi knowledge, tries to figure out how to neutralize the artifact and save the Ghost from being consumed by the Dark Side.