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 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.
Another idea is a crossover where the shinobi powers in Naruto's world are somehow combined with the unique abilities in the Rosario Vampire world. For instance, the vampires' super strength and speed could be enhanced with chakra control. This could lead to a new kind of battle system where characters from both universes have to learn to adapt and fight together against a common enemy, like a powerful dark force that threatens both their worlds.
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.
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.
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.
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 common theme is also the stranger being a new threat or a potential ally. They could be a hunter who has a different view on vampires compared to the existing ones in the story. So, the plot could revolve around whether the characters should trust this stranger or not. It often creates a lot of tension and interesting character dynamics.
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.
One plot idea could be Tsukune and Ria going on a journey together to discover a hidden power within the Rosario Vampire world. This could involve them facing various monsters and challenges along the way, and their relationship growing stronger as they rely on each other.