A third plotline could be a love triangle situation where Tsukune is caught between his feelings for other characters in the Rosario Vampire series and his growing closeness to Kahlua. This could lead to a lot of emotional drama, with Kahlua trying to win Tsukune's heart while Tsukune is conflicted about his own emotions.
For Tsukune, his character could be developed further by giving him more powerful vampire abilities. He could learn new techniques from Kahlua and use them to protect his friends. This would also make him more confident in the vampire world.
One common theme is her relationship with other characters in the Rosario Vampire universe. It could be her interactions with Tsukune, exploring a possible friendship or rivalry.
Fanfiction about Rosario Vampire Tsukune as a dragon might focus on his relationship with the vampire characters in a new light. Since he has this dragon power, the power dynamics between him and the vampires, especially Moka, could change. For instance, he might be able to protect her in ways he couldn't before, or there could be a new kind of tension due to his growing strength.
One popular theme is romance. Given the relationship between Tsukune and the other characters in the Rosario Vampire world, it's common to see fanfictions exploring the romantic relationship between Tsukune and Tsurara. It could be about how they first fall in love or overcome obstacles in their relationship.
One popular trope is the 'fish out of water' aspect as Tsukune, a human, is in a school full of monsters. Another is the development of his relationships, like his romantic relationship with Moka. There's also the trope of him having to prove himself in a world where he's initially seen as weak.
First, know the characters well. Tsukune is a complex character with his own growth arc. Make sure to stay true to his personality. Also, understand the world of Rosario Vampire, like the different monster species and their cultures. Then, come up with an interesting plot. It could be a new adventure for Tsukune or a different take on his relationships.
There are also fanfictions that focus on the backstories of Tsukune and Moka. Maybe exploring Moka's past as a vampire, her family history, and how Tsukune fits into all of that. Or looking at Tsukune's life before coming to the school and how it has influenced his relationship with Moka.
One common theme is the development of Tsukune's vampire powers. Writers often explore how he copes with his new abilities and how they change his relationships with the other characters.
Perhaps Tsukune, as a ghoul, starts to have a different view of the other supernatural beings at the academy. He could be drawn to the darker aspects of the ghoul world, which might put him at odds with the more light - hearted or morally upright characters. For example, he may have conflicts with the school's authorities who are trying to keep the peace between different species.
Some fanfictions give Tsukune the power of transformation. He can transform into a more powerful vampire form, with wings and increased physical capabilities. This not only looks cool but also gives him an edge in battles. It also shows his growth as a character, from a somewhat weak human - turned - vampire to a powerful force in the Rosario Vampire world.