In Star Wars, the Dark Side is often associated with power - hungry Sith. One interesting story is about Darth Vader's fall to the Dark Side. He was once a promising Jedi, Anakin Skywalker. But his fear of losing his loved ones led him to be seduced by Palpatine. His journey on the Dark Side is filled with internal struggles and great power, which ultimately led to his redemption at the end.
One interesting plotline could be the story of the Clone Wars side stories. They often show the individual battles and the moral dilemmas the clones faced. For example, some clones started to question their purpose and loyalty.
One possible story could be that Ahsoka, after facing a series of great losses and betrayals, starts to be tempted by the Dark Side. Maybe she discovers some ancient Sith knowledge that makes her question her Jedi teachings. She could struggle with the newfound power it offers and the darker emotions it stirs within her, like anger and vengeance.
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.
Perhaps there could be a story where Naruto's chakra is seen as a unique form of energy in the Star Wars universe. The Sith see it as a means to enhance their dark side powers. They manipulate Naruto, showing him false visions of his past failures in Konoha, which makes him angry and more susceptible to the allure of the dark side.
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.
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 interesting side story about Darth Vader is his encounter with a young Jedi in hiding. Vader was on a mission to root out any remaining Jedi, and he came across this youngling. Instead of immediately striking him down, he toyed with the boy, testing his skills. It was a display of Vader's power and his cruel sense of dominance.
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 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.
I would recommend 'Darkness Rising in the Galaxy'. This fanfiction has an interesting take on how the dark side could influence different planets and species in the Star Wars universe. It has some great action scenes and a complex plot that keeps you hooked from start to finish.