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.
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.
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.
You can try fanfiction websites like FanFiction.net or Archive of Our Own (AO3). These platforms usually have a wide variety of fanfictions. Just use the search function and type in 'oc slime x Rosario Vampire' and you'll likely find some interesting reads.
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.
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.
One idea could be that the male OC is a new student at the school. He has a special ability that he tries to hide at first. But as he gets to know the characters from Rosario Vampire, he gradually reveals it. For example, he might be able to control shadows and uses this power to help his new friends in a dangerous situation.
The Soul Reapers could be antagonists if they view Ichigo's vampirism as a threat to the balance between the living and the dead. They might try to hunt him down to eliminate this new, unregulated force.
In this fanfiction, the Church or religious authorities in Camelot could be the main antagonists. Vampires are often associated with the devil or evil in religious beliefs. So, they might see Merlin's transformation into a vampire as a sign of him being corrupted by dark forces. They would try to exorcise or destroy him, not understanding that he's still the same Merlin at heart, just with a different form and new powers.
I'm not sure if there is such a specific fanfiction where a character from Rosario Vampire is reading Naruto fanfiction. Fanfiction can be very diverse, but this seems like a rather unique and specific concept. It's possible that it might exist in the vast realm of fan - created stories, but it would be quite difficult to find without a more targeted search.
Tsukune and a powerful mage from the other fandom could be an interesting pair. Tsukune's vampiric powers combined with the mage's magic could create some really cool combat scenes and an interesting dynamic as they learn from each other.