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.
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.
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.
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.
The consequences would be huge. The Resistance would lose one of its most powerful allies. It would also mean a new threat to the galaxy. Rey's turn to the Dark Side could inspire others to follow her, leading to a resurgence of the Sith. The balance of the Force would be severely disrupted.
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.
If Rey turns to the Dark Side in fanfiction, one possibility is that she becomes a powerful and feared Sith. She might use her newfound dark powers to seek revenge on those who she believes have wronged her in the past. Maybe she would target the Resistance, seeing them as the ones who failed to fully understand or support her. Her connection to the Force could take a dark turn, with her using it for domination rather than protection.
She could be driven by a sense of betrayal. Maybe she discovers that the Jedi Order has been hiding something crucial from her, like a dark secret about her past or a false promise they made. This could shatter her trust in the light side and make her vulnerable to the temptations of the Dark Side.
It could be Darth Sidious. He is always looking for powerful allies. If he senses Ahsoka's potential and her growing disillusionment with the Jedi, he might manipulate her into joining the Dark Side. His cunning and power would make him a formidable antagonist in this scenario.