A common Rosario Vampire fanfic idea is a crossover. For instance, crossing over with another popular anime like Naruto. Tsukune could meet Naruto and learn some ninja techniques. Fanfics can also focus on a romantic sub - plot that was not fully explored in the original. Maybe a deeper exploration of the relationship between Tsukune and Kurumu, with more emotional moments and challenges they face together.
Some Rosario Vampire fanfic ideas involve time - travel. Imagine Tsukune being sent back in time to before he came to the school and having to make different choices. Also, fanfics about the backstories of the side characters are quite popular. Like exploring the past of Mizore and what made her the way she is. There are also fanfics that rewrite the entire plot with a different twist, such as if the monsters and humans were at war instead of co - existing at the school.
A great plot idea could be a tournament - style story. Characters from Rosario Vampire and the other fandom are forced to participate in a tournament where they have to fight each other. But as the tournament progresses, they start to realize that they have more in common than they thought and might need to work together to stop the tournament from being a tool for a greater evil. Another plot could be a time - travel crossover. A character from Rosario Vampire travels back in time in the world of the other fandom and has to prevent an event that would have a negative impact on both worlds. And, a simple yet interesting plot could be a cultural exchange where the characters learn about each other's cultures, like the different vampire clans in Rosario Vampire and the unique social structures in the other fandom.
Another concept is that Godzilla somehow ends up in the Rosario Vampire universe. This causes chaos as the local creatures and humans are not used to such a huge and powerful being. The main characters from Rosario Vampire then have to figure out a way to either send Godzilla back to his own world or find a way for him to co - exist in theirs.
A cool plot for a Rosario Vampire crossovers fanfic could be a time - travel scenario. The characters go back in time to the era when vampires were first created. They could meet the original vampire lords and learn about the true origins of their kind. This could also lead to a conflict as they might accidentally change the course of vampire history. Or perhaps there could be a crossover with a magical girl series. The Rosario Vampire girls could learn some new magical powers from the magical girls and use them to fight off a new threat that is immune to their normal vampire and monster abilities.
One idea could be a story where Yukari uses her magic to help Rosario in a difficult situation in the vampire world. Maybe Rosario gets caught in a power struggle among different vampire clans and Yukari's unique spells are the key to her escape.
One popular theme is the crossover of the two universes' powers. For example, a character from Rosario Vampire might gain the power of a Darksiders weapon. Another theme is the interaction between the characters from the two series. Maybe a character from Darksiders has to deal with the vampire society in Rosario Vampire. Also, adventure themes are common, where characters from both series team up to go on a quest.
One popular trope is the 'powerful - being - in - a - human - world' trope. Since both Rosario Vampire and Witchblade involve supernatural elements, fanfiction often plays with the idea of these powerful characters (vampires, witches, etc.) trying to fit into or interact with the normal human world. Another is the 'forbidden love' trope, as Rosario Vampire has a lot of complex relationships that can be explored further in fanfiction.
The theme of friendship within the Rosario Vampire world is also quite common in Moka - related fanfictions. Moka's friendships with other characters like Kurumu, Yukari, etc., are explored. How they support each other through various challenges in the school, whether it's dealing with other supernatural creatures or facing academic difficulties, is often a part of these fanfictions.
The action and adventure elements. There are battles against other monsters, challenges to overcome in the school environment. The story also has elements of mystery as Tsukune discovers more about the world of monsters and the secrets within the school. All these aspects combined make the Rosario and Vampire novel popular.
One idea could be that Batman ends up in the Rosario Vampire world while investigating a strange case. He has to deal with the vampires and other supernatural beings there while trying to find his way back home. Another could be a scenario where a character from Rosario Vampire travels to Gotham and causes chaos, and Batman has to figure out how to handle this new kind of threat that doesn't follow the normal criminal patterns. Maybe a vampire from Rosario Vampire becomes an ally of Batman in Gotham, and they team up to fight a greater evil that is affecting both their worlds.
One idea could be a story where the main character discovers a hidden power within themselves. Maybe they can control a new type of magic that no one in the Rosario Vampire world has seen before. This could lead to them having to face new enemies and also make new allies.