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.
One storyline could be a competition. For example, a race or a mission where Anakin and his rival are pitted against each other to see who can achieve a goal first. This could create a lot of tension and show different aspects of their characters.
A rival could be motivated by envy. The Straw Hat Pirates are known for their fame and the bonds they share. Another pirate or character might envy their camaraderie and the way they are able to overcome difficulties together. So, they want to break the Straw Hat Pirates apart and prove that their own way is better.
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.
One great rival could be a character with a powerful Devil Fruit ability. For example, a character who has a Logia - type Devil Fruit that allows them to turn into a natural element like lightning. This would make them very difficult to defeat in combat as they could dodge attacks easily. They could also use their element - based powers to attack the Straw Hat Pirates in unexpected ways, such as sending bolts of lightning towards their ship.
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.
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.
In a One Piece fanfic, an OC (Original Character) as Luffy's rival could be really interesting. This OC might have a unique Devil Fruit ability that directly opposes Luffy's powers. For example, if Luffy has the Gum - Gum Fruit which gives him stretchy abilities, the rival could have a hardening or shrinking ability. This would create intense battles between them as they fight for different ideals on the Grand Line.
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.