A possible plot is Anakin being an outcast among the Sith. Even though he has turned to the dark side, his past as a Jedi makes other Sith distrust him. He then has to prove himself worthy of being a true Sith. He might take on dangerous missions alone, like infiltrating a Republic base to steal important Jedi artifacts, all while trying to earn the respect of his new Sith brethren.
One idea could be Anakin as a Sith having to face his former Jedi allies. He might struggle with his new identity while still having remnants of his Jedi self. For example, he could be sent on a mission by the Sith to destroy a Jedi stronghold but find himself hesitating when he sees his old friends there.
Perhaps a fanfiction where Anakin, as a Sith, is sent on a mission by Palpatine. But on this mission, he meets a young Force - sensitive who reminds him of his own past. This makes him start to have doubts about the Sith ways. He begins to secretly teach the young one about the Force in a more balanced way, all while trying to hide it from Palpatine.
The SI could be a smuggler who crosses paths with Anakin during one of his missions. They team up and the SI shows Anakin a different side of the galaxy, one that's not all about the Jedi and Sith. This could make Anakin question his place in the galaxy and his loyalty to the Jedi Order. Their adventures together could range from dealing with pirates to uncovering ancient secrets.
One plot idea could be Rey being tempted by the dark side and Sith teachings in a more psychological way. Maybe she starts having visions of past Sith Lords who try to convince her that the power they offer can help her protect the galaxy more effectively. Another could be a story where Rey discovers a hidden Sith artifact that has the power to either corrupt or be redeemed, and she has to make a choice about what to do with it. A third idea is that Rey has to face a new Sith threat that is related to her own past in some way, like a long - lost Sith relative or a Sith cult that was involved in her family's history.
One idea could be a young Jedi who falls to the dark side and starts to gather a harem of Sith apprentices, each with their own unique skills and backstories. They could all be on a quest to find an ancient Sith artifact that is said to grant ultimate power.
One idea could be exploring what would have happened if Anakin had a different mentor during his fall to the dark side. Maybe a Jedi who was more in tune with his emotions could have guided him better.
Another idea is that the Sith Empire decides to use a new form of Sith magic during the Clone Wars. This magic starts to corrupt the planets it is used on, and the Jedi have to find a way to stop it. The story could show the different reactions of various species to this new threat. For example, some might be more susceptible to the corruption while others are more resistant. The Jedi would have to form alliances with these different species to combat the Sith Empire's new weapon. This would involve a lot of political maneuvering as well as physical battles. It could also explore the nature of the Force and how the Sith's use of it in this new way affects the balance.
One idea could be that Anakin's spirit somehow guides Rey in her Jedi training. Maybe he appears to her in visions and imparts his unique knowledge of the Force, especially the more unorthodox aspects he learned during his time. Another plot could involve Rey finding an ancient artifact that was once connected to Anakin, and through it, she uncovers secrets about his past that help her in her fight against the new Sith threat.
As he spends more time as a Sith, he might start to feel a sense of emptiness. He realizes that the power he sought isn't fulfilling. He could then have a moment of self - reflection. Maybe he meets a character who challenges his beliefs, like a young Force - sensitive who still believes in the good in the galaxy. This makes him question his path and start to seek a way back from the dark side.
Maybe a story where Ahsoka has a premonition about Anakin's fall to the dark side. She tries to prevent it in secret, which leads to all sorts of dangerous situations and moral dilemmas as she struggles with whether to tell the Jedi Council or handle it on her own.
Well, a great storyline could involve an OC Sith who was once a Jedi padawan but was seduced by the dark side. In 'Star Wars The Clone Wars', this character could be on a journey to destroy their former Jedi friends while also dealing with the internal conflict of their new Sith identity. They may struggle with memories of their Jedi training and the relationships they had before turning to the dark side.