Often, a Jedi - turned - Sith OC fanfiction will use the plot device of a mentor figure in the Sith. This mentor could be guiding the OC down the dark path, teaching them Sith secrets and manipulating them for their own ends. Also, the OC might have a personal vendetta against the Jedi Order that led them to turn, such as feeling betrayed by the Order in the past.
To write a great Jedi - turned - Sith OC fanfiction, focus on character development. Give your OC unique traits and motives. For instance, if they were a healer Jedi before, how does that change when they turn? Also, create interesting relationships. Have them interact with other Sith, Jedi who are hunting them, or even civilians caught in the crossfire. The interactions should drive the story forward.
One common theme could be the struggle between the light side (jedi) and the dark side (sith) within a relationship. It might explore how the two opposing forces interact on a personal level.
One common plot device is the discovery of his true identity. It's often a big moment in the story where the oc prince realizes he's a Targaryen. Another is the use of prophecy. Maybe there's a prophecy about him that drives the plot forward. For example, he might be the one to restore the Targaryen glory or something like that.
One frequent plot device is the OC being a new arrival in the setting of the opera. This allows for a lot of discovery and explanation of the world to the reader through the OC's eyes. And then there's the idea of the OC being a catalyst for change in Erik's life. Maybe the OC's presence makes Erik question his actions and motives more than he ever did before in the fanfiction.
One common plot device is the protagonist having unique abilities from their past life. For example, they might have knowledge of advanced fighting techniques that don't exist in the RWBY world yet. Another is the struggle to fit into the new world while still holding onto their old self. They may find it hard to adjust to the different social norms and combat styles. Also, the discovery of new relationships that are influenced by their reincarnated state. They could form unexpected alliances or rivalries because of their different perspective.
A common plot device is the OC having a mysterious past. This draws Arno's attention and makes him want to uncover the secrets. For example, the OC might be hiding her identity as a member of a secret society.
Another theme is secrecy. Their relationship has to be hidden from their respective orders. In many fanfics, they have to meet in secret places and communicate without being detected. It adds a sense of mystery and danger to their love story. In a story I read, the Jedi and Sith lovers had to use secret codes just to exchange messages.
One common theme is the struggle between light and dark. Since Jedi represent the light side of the Force and Sith the dark, their love is often a battle between these two opposing forces. Another theme is forbidden love. Their respective orders would not approve of their relationship, so they have to keep it a secret. Also, redemption is a theme. Sometimes the Sith in the relationship may be redeemed through the love of the Jedi.
Redemption is another theme that can be found. Maybe the female Sith has a change of heart because of her relationship with the male Jedi, or the Jedi has to face his own inner demons when dealing with his feelings for the Sith. These themes add depth to the 'Male Jedi x Female Sith' fanfiction stories.
A common theme is the struggle between light and dark. The Jedi represents the light side of the Force, and the Sith the dark. Their romance is often a battle of these two opposing forces within themselves and in their relationship. Another theme is redemption. Sometimes the Sith in the relationship is shown as being redeemed through the love of the Jedi.