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.
A possible story idea could be that your OC Sith discovers an ancient Sith prophecy. They believe it will lead them to ultimate power. So, they start a quest to find the artifacts related to this prophecy during the Clone Wars. This could involve them infiltrating Jedi strongholds and causing chaos.
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.
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.
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.
Perhaps a Sith Lord is kidnapped by a mysterious cult that worships an ancient and powerful force entity. The Sith Lord has to use his wits and dark side powers to escape while also trying to figure out the cult's motives. This could involve battles with strange cultists and uncovering dark secrets.
Perhaps a Sith Inquisitor is sent on a mission to hunt down a Jedi who has some knowledge that could undermine the Sith order. Along the way, he starts to question the Sith teachings. He could meet some interesting characters during his hunt that make him see things differently. For example, he might encounter a group of rebels who are fighting for freedom, and this makes him wonder if the Sith way is truly right. As he gets closer to the Jedi, he has to battle his own inner demons as well as the physical threats that come his way.
A Sith Lord in your Star Wars OC fanfiction could have a lair on a desolate planet. It could be filled with ancient Sith artifacts. When it comes to their motives, it could be as simple as wanting to rule the galaxy or more complex like seeking revenge for something that happened in the past. You can also give them a distinct voice, like a deep, raspy tone that sends shivers down the spines of those who hear it.