The SI could also be a love interest for Anakin. In a fanfiction, this could be a very different kind of relationship compared to what he had with Padmé. This relationship could either be a positive influence, giving Anakin more stability and a reason to stay on the light side, or it could be a source of conflict, especially if the SI has their own secrets or agendas.
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 powerful Nightsister could be a great OC. The Nightsisters have their own connection to the dark side of the Force. An OC Nightsister could use her magic and knowledge of the dark side to interact with Anakin. She might offer him new ways to use the Force, like some of the unique spells and abilities that Nightsisters possess.
One common theme is redemption. Since Anakin's story in the official Star Wars lore is about his fall and redemption, many fanfictions with OCs explore this further. The OC might be the catalyst for Anakin's redemption or be involved in his journey in some way.
Often, Anakin's development in these fanfictions involves him exploring different aspects of the Force. He might discover new Force abilities that were not part of the original story. This new knowledge helps him to dominate in various situations, whether it's in combat or in diplomatic negotiations. Also, his relationships with other characters, like Obi - Wan or Padmé, are often explored more deeply, which in turn shapes his character growth.
In fanfiction, Anakin's character can be expanded through alternate timelines. Let's say a fanfic has him not turning to the dark side. We can then see how his character would have developed as a powerful Jedi without the influence of the Sith. It also allows for exploring his relationships with other characters like the clones in ways that the movies didn't fully cover. Writers can create scenarios where Anakin has to lead the clones in different battles and how that shapes his leadership skills and his view of the war.
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 plot idea could be that your SI OC is a young Force - sensitive being discovered by the Jedi Order, but they have a connection to an ancient Sith prophecy. The Jedi must decide whether to train them or fear their potential for darkness. Another idea is that your OC is a bounty hunter who gets caught in the middle of a power struggle between the Hutts and a new criminal syndicate. They have to use their wits and skills to survive and maybe even tip the balance of power.
One great Dark Anakin fanfiction could be about his internal struggle. It might show how the dark side slowly creeps in as he faces difficult decisions in the Clone Wars. For example, a story could focus on a mission where he's tempted to use excessive force to achieve victory, and how that starts to change his mindset towards the dark side.
To keep Anakin's character true, writers pay attention to his relationship with the Force. His connection to the light and dark side is crucial. In a crossover, if he's in a world without the Force as we know it, his struggle to understand this new power - less world while still having his internal Force - related battles is a way to stay true to his character. For instance, if he crosses into a world of technology, he might be frustrated by the lack of a spiritual power like the Force and try to find equivalents.
In some fanfictions, Anakin's past as a slave greatly influences his character during The Clone Wars. It might be explored how his experiences of being a slave shape his views on freedom and power. For example, he could be more empathetic towards the enslaved in certain storylines, or his fear of losing his new - found freedom could drive some of his actions during battles and political situations in the Clone Wars era.