Zero is another great choice. His struggle with his vampire side while trying to protect humans is a rich area for storytelling. A fanfic could delve deeper into his past, his family's history, and how that affects his present actions and relationships. For example, how his hatred for vampires is tempered by his feelings for Yuki.
Yuki is a very popular character to focus on. She has a complex backstory, being raised among vampires and having a special connection to both the vampire and human worlds. Her relationship with Zero and Kaname also offers a lot of material for fanfic writers to explore, such as her inner conflict between her love for both of them and her duty to maintain the peace at Cross Academy.
Rose Hathaway is a very popular choice. She's strong, independent, and has a complex personality. A fanfic could explore more of her backstory, like her early training days or her relationship with her mother.
Yuki is also a beloved character in fanfiction. Her unique position between being human - like and having vampire traits can be further explored. In Pureblood fanfiction, she might interact with Purebloods in ways that are different from the original story, such as getting involved in Pureblood politics.
Zero is often very popular. His complex character, being a vampire hunter with vampire blood himself, makes him full of contradictions and depth. Fans are intrigued by his internal struggle between his human side and the vampire instincts within him.
Well, Armand is often a central character in fanfiction. He has a long and mysterious history, and his relationship with other vampires, especially Lestat and Louis, can be delved into deeply. His powers and his leadership within the vampire community are also interesting aspects to focus on. Claudia is another popular character. Her unique situation as a child vampire and her relationship with her 'fathers' Louis and Lestat can lead to some very emotional and complex storylines.
Batman is obviously a very popular choice. Writers often like to delve deeper into his psyche, especially after the events of 'The Dark Knight Rises'. They might explore his internal struggles with coming back to the role after a long break or his thoughts on how to better protect Gotham in the future.
Elena is always a popular choice. She's the central character in the original series, and there are so many ways to explore her character further in fanfiction. Stefan is also a great character to focus on. His struggle with his vampire nature and his love for Elena can be delved into more deeply. Damon is another one. His bad - boy charm and complex personality can add a lot of spice to a fanfiction story.
Shikadai is quite popular. His laid - back yet intelligent nature makes for an interesting character to write about. Inojin and Cho - Cho are also good choices. You can focus on their growth as ninja, their relationships with their parents' generation, and how they contribute to the new ninja world in the 'naruto gaiden fanfic'.
Nagisa Shiota is also very popular. Fanfics could focus on his growth as an assassin and how his initially meek personality evolves over time. His unique skills and his friendship with other classmates can be great elements to build a story around.
Louise is a very popular character to focus on. She has a complex personality, being proud yet often insecure. Her growth as a mage throughout the series can be further explored in a fanfic, especially her relationship with her magic and how it evolves.
Sasuke is always a popular choice. His complex backstory and his journey of redemption can be explored further in fanfics. Writers can delve into his internal struggles, like his feelings towards his clan and his relationship with Naruto in more depth.