A new Sith Lord could be the main antagonist. This Sith Lord could have been in hiding for years, waiting for the right moment to emerge. When Rey starts to show signs of turning to the dark side, he sees her as a perfect tool or a potential apprentice. But when Rey doesn't conform to his plans completely, he becomes her enemy. He could be extremely powerful and have a whole legion of dark side followers at his command.
Kylo Ren could still be the main antagonist even if Rey turns to the dark side. He might see her as a threat to his own power within the dark side hierarchy. Maybe he wants to keep her in check or even destroy her to maintain his dominance.
It could be the remaining Jedi. Since Rey is turning, the Jedi who still uphold the light side would see her as a threat. They would try to stop her from causing more harm with her newfound dark side powers.
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.
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.
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.
A plot twist could be that Rey has a dark heritage she was unaware of. When she discovers this, she feels a pull towards the Dark Side. She may struggle with this newfound knowledge at first, but eventually give in. In the story, her transition to the Dark Side could cause chaos in the Star Wars universe. The Resistance would be shocked and some might try to bring her back. However, the Dark Side could give her new powers and a different perspective on the Force. She could then start to reshape the Sith ideology, creating a new form of the Dark Side that is more in line with her own beliefs, leading to a whole new conflict in the galaxy.
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.
In Star Wars fanfiction, Dark Rey is a fascinating concept. It allows for a different take on the character of Rey. Writers can explore her journey to the dark side, perhaps due to feelings of abandonment or a thirst for power. It gives a chance to see how her relationships, like with Kylo Ren, would change in a darker context.
A new, mysterious dark side character could also be introduced in the fanfiction to hypnotize Rey. This character could have a unique power that allows them to manipulate minds, and they target Rey because of her strong connection to the Force. They might see her as a threat or a tool to be used for their own evil plans.
One idea could be Rey being lured to the dark side by a powerful Sith artifact. Maybe she discovers an ancient Sith holocron that shows her a version of the galaxy where the dark side rules supreme and she starts to believe that it's the only way to bring true order. Another plot could involve Rey being manipulated by a dark side user who preys on her insecurities about her past and her power.
It could be a new, powerful dark side user created specifically for the fanfiction. This character might have a vendetta against the Skywalker family and sees turning Leia as a way to get revenge on Luke and Anakin. They could use Leia's insecurities and her desire to protect her loved ones against her.