An interesting plot could involve the OC slime being a sentient experiment from another dimension. In the Rosario Vampire world, it starts to develop emotions and a sense of self. It could be drawn to Moka because of her unique vampire - human hybrid nature. As it gets closer to the group, it has to learn about their world's rules and values. It might even accidentally create chaos by using its slime - based powers in unexpected ways, like accidentally covering the entire school in slime during a fight, which then leads to some comical and also some more serious situations as they try to clean it up and deal with the consequences.
There could be an external force from the slime's original world that is trying to bring it back by any means necessary. This force could be very powerful and not care about the consequences in the Rosario Vampire world. They could be the ones behind some of the strange events that start to happen in the school, like summoning dark creatures to attack the school in order to force the slime to reveal itself so they can capture it and take it back.
The combination of an original character (oc) as a slime and the Rosario Vampire universe can be really cool. It allows for unique interactions and storylines that might not be possible with the original characters alone. The slime concept can bring in elements of transformation and different abilities, which can add a fresh twist to the Rosario Vampire world.
In a Naruko x Rosario Vampire crossover fanfiction, the plot could center around a tournament. The idea could be that both the ninja world and the Rosario Vampire world participate in a grand tournament to prove which world has the stronger fighters. Naruko would be one of the representatives from her world. She would face off against the various characters from Rosario Vampire. This would lead to some interesting battles and also some unexpected friendships as they learn about each other's cultures and fighting styles.
One idea could be that the main characters from Rosario x Vampire end up in an alternate universe where they have to face new challenges based on the rules of the other fandom. For example, they might encounter different types of supernatural beings with unique powers.
The OC could be a half - human, half - werewolf. They are trying to find a place where they belong, as they are not fully accepted in either the human or werewolf worlds. At Yokai Academy, they meet different characters from Rosario Vampire. Their journey of self - discovery could involve learning to control their powers, dealing with the politics between different monster species, and finding true friendship and love within the academy.
One interesting plot could be that a vampire from the Rosario Vampire world somehow ends up at Hogwarts. Maybe they're initially mistaken for a dark wizard because of their vampiric abilities. They could then form an unlikely friendship with Harry Potter and his friends, and together they have to face a new threat that combines the magic of the Harry Potter universe with the unique powers and challenges of the vampire world.
A great plot could involve a powerful enemy that has the ability to drain the 'viewtiful' energy from Joe. Tsukune and his friends notice the weakened Joe and decide to help. They all train together, combining their different powers and skills. The story could then build up to an epic showdown where they all have to use their combined strength to defeat the enemy and Joe regains his full 'viewtiful' power.
One possible plot could be that the Fairy Tail guild members end up in the Rosario Vampire world. Natsu could mistake the school for a new guild and start causing chaos while trying to find a 'big bad' to fight. Meanwhile, Tsukune from Rosario Vampire could be intrigued by the magic of the Fairy Tail members and try to learn from them, leading to a mix - up of their two worlds' powers and cultures.
One idea could be that the OC is a new student at the school and has a unique power that Moka is initially suspicious of. But as they interact more, they find they have a common goal of protecting the school from a new threat. For example, a group of powerful vampires from outside are trying to take over the school. The OC and Moka team up, and during this process, they develop a deep friendship that could potentially turn into something more.
The OC could be a werewolf. In the Rosario Vampire universe, there's already a complex web of relationships between different supernatural beings. Mizore, as a yuki - onna, might have an initial conflict with the werewolf OC due to the natural differences between their species. But over time, they could overcome these differences. Maybe they find out that they both have a love for a certain type of food that is hard to find in their school cafeteria, like a special type of pudding. This shared taste could start to break down the barriers between them.