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.
One of the most popular crossover scenarios might be the ones that involve unique power combinations. For example, in a Naruto and Highschool DxD crossover, Naruto's jutsu combined with the devil powers in Highschool DxD could create really interesting battles. In a Rosario Vampire and Highschool DxD crossover, the different types of supernatural beings from both series could interact in new and exciting ways. Also, crossovers that explore the different school settings and how the characters from each series adapt to them are quite popular.
Maybe Naruto forms an alliance with some of the main characters in Rosario Vampire. He could use his ninja skills combined with his vampire abilities to fight against common enemies. Like, if there's a powerful dark force threatening both the vampire and human worlds, Naruto could be a key player in the defense.
Well, he could fit in as a unique addition to the cast. His ninja abilities would be quite different from the normal powers in the Rosario Vampire world. He might be seen as a wildcard, with his speed and combat skills being a big advantage.
One idea could be a crossover where a character with the Sharingan from Naruto somehow ends up in the world of Rosario Vampire. They could use their Sharingan abilities to detect and combat the various supernatural threats in the Rosario Vampire world, like the vampires and werewolves. Maybe they'd form an alliance with Tsukune or Moka.
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.
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.
Well, a cool storyline might be that the villains from the Ben 10 universe somehow find their way to the Rosario Vampire world. And they start causing trouble by trying to take over the school or capture the vampires for their own evil purposes. Ben then has to team up with Tsukune and the other students to stop them. Or perhaps there could be a story where Ben accidentally gets turned into a vampire - like creature because of some magic in the Rosario Vampire world, and he has to figure out how to reverse it while still using his alien powers.
The Sharingan could be used to see through illusions created by the vampires in Rosario Vampire. Since vampires often have powers related to charm and mind - control, the Sharingan's ability to break illusions would be very useful. For example, if a vampire was trying to hypnotize a character from Naruto, the Sharingan could counter it.