One interesting plot twist could be a character from Rosario Vampire getting transported to the Ranma universe and having to adapt to its unique martial arts and gender - changing situations. For example, Moka could end up in Furinkan High School and have to deal with Ranma's crazy antics. Another plot twist might be a fusion of the two worlds' magic systems. Maybe the nekos from Rosario Vampire learn some of Ranma's martial arts techniques and combine them with their own powers to create new and powerful attacks. A third plot twist could involve a love triangle between a character from each series and a new character created in the crossover. This would add a whole new layer of drama and conflict to the story.
The martial arts skills of Ranma could be combined with the supernatural abilities of the Rosario Vampire characters. For example, Ranma's quick and powerful punches and kicks could be enhanced by Moka's vampire strength. Or, Ranma could use his agility to dodge attacks while using the magic of the Rosario Vampire characters to counter - attack. Another way their abilities could combine is through teamwork. The nekos from Rosario Vampire could use their speed and stealth to set up attacks for Ranma, while Ranma could use his combat experience to protect the more vulnerable characters from Rosario Vampire.
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 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.
The unique powers. In 'bleach', characters have zanpakutos and shinigami powers, while in 'rosario vampire' there are unique yokai abilities. When these are combined in fanfiction, it makes for really cool battles and power - up scenarios.
One common theme is power combination. For example, Naruto's chakra - based powers combined with the unique abilities of the Rosario + Vampire characters like Tsukune's vampiric powers. Another theme is cultural clash. Naruto comes from a ninja - based culture, and when he enters the Rosario + Vampire world, there are differences in values, social structures, and fighting styles that create interesting scenarios.
It's likely that Ben with the Omnitrix would be one of the most powerful. He has access to a wide variety of alien forms, each with their own unique abilities. For example, Heatblast could deal with cold - based threats in the Rosario Vampire world, and XLR8 could outrun any fast - moving vampires or enemies. However, some of the powerful vampires like Alucard in Rosario Vampire also have incredible strength, speed, and magical abilities. So it could be a close call between Ben in his most powerful alien forms and the top - tier vampires.
There is a fair amount of 'gaara x rosario vampire crossover fanfiction'. These crossovers can be really interesting as they combine the unique powers and personalities from both Naruto (where Gaara is from) and Rosario Vampire. For example, Gaara's sand abilities interacting with the vampire and monster - filled world of Rosario Vampire can lead to some very creative and exciting storylines. Some fanfictions might also explore how Gaara would fit into the school - like setting of Rosario Vampire, or how the Rosario Vampire characters would react to his rather stoic and powerful nature.