A possible plot could be that a powerful artifact in the Rosario Vampire world is being targeted by both Hollows and some rogue vampires. Ichigo and Tsukune find themselves on a quest to protect it. Along the way, they encounter various challenges and meet different characters from both universes. The cultural shock between Ichigo's world of Soul Reapers and Tsukune's world of monsters and humans could also add some comedic and interesting elements to the story.
Begin with a prologue that sets up the situation. Maybe there are strange energy readings in both the Bleach world and the Rosario Vampire world that are somehow linked. Ichigo is sent to investigate in his world, and as he gets closer to the source, he is transported to the Rosario Vampire world. Once there, he is greeted by the chaos of the Yokai Academy. You can then start to introduce the different characters and their reactions to Ichigo's sudden appearance. This will slowly build the foundation for the rest of the story.
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.
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.
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.
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 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.
They could be in a situation where they are both trapped in a magical maze. They have to rely on each other's unique abilities to find a way out. Moka's strength and Gyokuro's intelligence would be put to the test. This would be an interesting exploration of their characters as they are forced to cooperate.
One idea could be a crossover where Naruto characters end up in the Rosario + Vampire world. For example, Naruto could be mistaken for a powerful yokai due to his chakra. Sakura might study the unique magic in the Rosario + Vampire world. Another could be a vampire in the Naruto world, using their powers in a different way than the normal jutsu.
Fanfiction could also center around Mizore and Tsukune training together. Since they both have different powers, they could learn from each other. Tsukune could teach Mizore about human resilience and determination, while Mizore could show Tsukune how to better control his powers, especially when it comes to the more vampiric aspects. Their training sessions could be filled with both comical and intense moments.
In a fanfiction about pregnant Mizore, the story could revolve around how she prepares for the arrival of her baby. This could include things like finding a safe place in the Rosario Vampire world for the baby to grow up, making sure she has the right support system, and also how she balances her pregnancy with her duties or relationships within the vampire and monster community.