Betrayal can be a theme as well. Given Anakin's complex character arc in the official Star Wars story, fanfiction writers may explore how an OC could be involved in a situation of betrayal, either being betrayed by Anakin or vice versa. This adds a lot of drama and tension to the story.
One popular 'anakin x oc fanfiction' is 'A New Dawn' which beautifully weaves the story of Anakin and an original character. It delves into their relationship, how they meet and how they grow together. The OC in this story has a unique backstory that complements Anakin's character well.
One common theme is redemption. Often, the OC plays a role in Anakin's journey towards redemption, either by believing in him when others don't or by showing him a different way of looking at things. For example, in some fics, the OC might help Anakin see that there are alternatives to the dark path he's on.
First, create a unique original character (OC). Develop their backstory, personality, and their reason for having a rivalry with Anakin. For example, if your OC is a Jedi, they might have different views on the Force or the Jedi Code compared to Anakin. Then, think about the setting and the events that will fuel this rivalry. It could be a competition for a certain mission or a difference in ideology within the Jedi Order. Start with a strong opening that immediately shows the tension between the two characters.
The OC master could be a hermit who has been living in isolation for years, developing a unique understanding of the Force. He or she might have witnessed the downfall of the Jedi Order in the past and wants to prevent Anakin from making the same mistakes.
The setting could be a major element. It might be set in a different time or place compared to the original Star Wars universe. Maybe it's during a period of relative peace, and the training is more about self - discovery for Anakin rather than combat - oriented. Also, the powers and abilities that the OC imparts to Anakin can be important. It could be new Force techniques or ways of thinking about the Force that we haven't seen before in the official stories.
Perhaps it's a non - Force - sensitive character who has skills in other areas that are important in the Star Wars universe. For example, a great pilot or a brilliant strategist. This person could be getting more attention or praise than Anakin in certain situations, and thus be seen as a rival.
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.
One great Anakin and OC fanfiction is 'A New Path'. It explores a unique relationship between Anakin and an original character who is a Jedi healer. Their interactions are filled with emotional depth and the story has some really interesting plot twists.