One plot idea could be a tournament - style competition where characters from both Naruto and Rosario Vampire universes participate. The harem aspect could come in as different characters form alliances and friendships. Another idea is a mystery where they have to solve a strange phenomenon that is affecting both worlds. Maybe some characters are losing their powers and they have to figure out why.
One interesting plot idea could be that Naruto somehow gets transported to the Rosario Vampire world and has to adapt to its unique rules. He could end up forming a harem with the female characters from the Rosario Vampire series, like Moka. Maybe he uses his ninja skills to protect them from other threats in that world.
Yes, there are likely some. You can start by looking on fanfiction platforms like FanFiction.net or Archive of Our Own. Search for the 'Naruto Rosario Vampire Harem' crossover and then filter through the results to find ones that don't have inappropriate or 'lemon' content. Some authors focus more on the adventure and character interactions rather than adult - themed elements.
One interesting plot idea could be Tsukune having to protect his harem members from a new powerful vampire threat that has emerged in the school. This would involve a lot of action scenes where Tsukune uses his growing 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.
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.
Well, a simple plot could be that Female Tsukune is new to the school and accidentally catches the eyes of several interesting characters, thus forming a harem. They each try to court her in their own unique ways. A vampire might use his charm and knowledge of the night to impress her, a witch could use magic to create beautiful things for her. As the story progresses, Tsukune has to figure out who she really likes among them while also dealing with the jealousy and drama that comes with a harem situation.
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.
One interesting plot twist could be when Naruto, as a vampire, discovers a hidden power within himself that is related to the Rosario Vampire lore in a completely unexpected way. Maybe it's a power that only activates when he's in the Rosario Vampire world and it changes the course of the story.
One idea could be that Naruto is somehow transported to the Marvel universe and his unique abilities draw the attention of several female characters. They form a sort of harem around him as they are intrigued by his ninja skills and his never - give - up attitude. Another could be that a magical or scientific accident combines elements of the Naruto world and the Marvel world, creating a situation where a harem forms around Naruto as he navigates this new and strange combined reality.