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.
You can try looking on fanfiction websites like FanFiction.net or Archive of Our Own (AO3). These platforms have a large collection of fanfictions from various fandoms, including Rosario Vampire. Just use the search bar and type in 'Rosario Vampire Tsukune and Tsurara' to see the available stories.
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.
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 popular theme is the development of their relationship. Fans love to read about how Kurumu's initial infatuation with Tsukune grows into a deeper, more meaningful connection. Another theme is adventure. They often go on quests or face challenges together, like fighting against powerful enemies in the Rosario Vampire universe.
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.
One common theme is power struggles. Since Alucard is a very powerful vampire and Tsukune is a human - turned - vampire with his own unique abilities, fanfictions often explore how they interact in terms of power. For example, they might have to team up against a greater threat while also dealing with their own differences in power levels.
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.
One main theme could be power. With Tsukune as a god, power dynamics are likely to be a big focus. Another might be transformation, not just in terms of Tsukune's new status but also how it affects the other characters around him.
Another theme might be his relationships. Maybe in these bashing fanfictions, his relationships with the female characters are misconstrued or made to seem one - sided in a negative way. For example, his bond with Moka could be portrayed as him being clingy or unworthy. Also, his attempts to fit into the monster school could be shown as him being incompetent rather than the plucky underdog he is in the original story.
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.