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.
In terms of character interactions, Naruko would bring her energetic and determined personality to the Rosario Vampire world. She would be eager to learn about their magic and powers. The vampires, on the other hand, would be fascinated by Naruko's ninja abilities. Inner Moka could challenge Naruko to a friendly sparring match to test their strengths. Tsukune would try to mediate between them if things got too intense. And the other characters from both worlds would gradually start to understand and respect each other as they interact more. For instance, Naruto's loyalty to his friends could inspire Tsukune to be more assertive in protecting his friends in the Rosario Vampire world, and the Rosario Vampire characters' acceptance of their differences could teach Naruko's friends a lesson about tolerance.
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.
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.
A plot could be that Tsukune has a strange dream about Akasha one night. In the dream, she is in some kind of danger. When he wakes up, he can't shake off the feeling. So, he starts to search for any signs of trouble around Akasha. As he gets closer to her, he finds that there are some dark forces at work that are indeed targeting Akasha, and he has to use his own unique abilities (maybe some new ones he has developed) to protect her.
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.
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.
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.
In this type of fanfiction, it could start with a strange magical artifact in the Naruto world that has a connection to the Rosario world. When Naruto touches it, he and some of his comrades are transported to the Rosario world. There, they are mistaken for enemies at first. But as they show their good intentions, they start to work together with the Rosario characters. They might learn about different forms of power like the Rosario vampires' unique abilities and combine them with their own ninja skills to face challenges in this new world.