One possibility is that she becomes disillusioned with the Rebellion. Maybe she discovers some dark secrets within the Alliance that make her question their motives. For example, if she finds out that the Rebellion has been using unethical tactics in the fight against the Empire. This could push her towards the dark side as she feels betrayed by those she once trusted.
She becomes more ruthless. Instead of the diplomatic and empathetic Leia we know, she starts to use fear and intimidation to get what she wants. For instance, she might not hesitate to sacrifice innocent lives if it means achieving her goals.
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.
One possible plotline could be that after facing great losses and betrayals, Leia starts to question the light side's values. Maybe the death of a close friend or the destruction of something she holds dear makes her vulnerable to the temptations of the dark side. She might then begin to explore the power that the dark side offers as a means to gain control and revenge.
One possible scenario could be that Leia is manipulated by a powerful dark side user. Maybe through false promises of power or a way to protect her loved ones in a distorted way. Another could be that she becomes disillusioned with the light side due to some great loss or perceived betrayal within the light side forces.
The power balance would be severely disrupted. The Rebellion would lose one of its most important leaders, and the Empire or any other dark side - aligned forces would gain a significant advantage. Her knowledge of the Rebellion's strategies and bases could be used against them.
In a 'Harry Potter turns to the Dark Side' fanfiction, the plot might involve Harry being influenced by a powerful dark object. For example, a cursed artifact that makes him see the Dark Arts in a different light. He could start to believe that the ends justify the means and that the Dark Side has been misrepresented all along. As he delves deeper into the dark side, he begins to turn against his old friends, thinking they are too naive and weak to truly change the wizarding world.
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 fanfiction, if Ezra Bridger turns to the dark side, his character would experience a significant transformation. His once - noble Jedi - like qualities would be replaced by a more self - serving and power - hungry nature. His relationship with Sabine would become extremely complicated. Sabine might initially be in denial about his turn, thinking there's still good in him. But as time passes, she'd have to face the reality. If Sabine turns to the dark side, her Mandalorian values could be perverted. Her loyalty could shift from her friends to some dark power. And Ezra would have to decide whether to save her or stop her, which would be a huge internal struggle for him.