One possible aspect of a Naruto and Monster High fanfic could be the characters' interactions. For example, Naruto's determination and positive attitude might clash or blend with the unique personalities of the Monster High characters. Maybe Naruto could teach them about his ninja way, while the Monster High students introduce him to their spooky yet fun world.
One interesting storyline could be that Naruto and his friends are transported to the Monster High world. They could team up with the Monster High characters to face a common threat. For example, a powerful demon that is causing chaos in both the Ninja world and the Monster High world. Another could be a cultural exchange where Naruto shows the Monster High students his ninja skills and they in turn show him their unique monster abilities.
One possible plot could be that Ben 10's aliens end up in the Monster High world. For example, Four Arms could get into a friendly strength - testing competition with some of the stronger monsters at Monster High like the werewolves. It would be a clash of different kinds of powers and cultures. Ben, in his human form, might also have to figure out how to fit in and make friends with the unique students at Monster High.
Maybe there's a magical mishap that causes some of the characters' personalities to swap. A Monster High character could suddenly find themselves with the storybook - bound personality of an Ever After High student and have to navigate their new identity while trying to figure out how to reverse the spell. This would lead to some really funny and interesting interactions as they try to fit into a role that's completely different from their own.
In this fanfic, let's say a magical portal opens up between the world of Mystery Skulls and Monster High. One of the Mystery Skulls' members accidentally falls through it and lands right in the middle of Monster High's campus. The students are both curious and a bit scared at first. But soon, they start to get along. Lagoona Blue offers to show the Mystery Skulls member around the school, introducing him to the various monster students. Meanwhile, the other Mystery Skulls are desperately trying to find a way to open the portal again to rescue their friend. Back at Monster High, the new arrival starts to share some of his knowledge about the spirit world with the students. They find it fascinating and start thinking about how it could be applied to their own studies of the supernatural. Eventually, the Mystery Skulls manage to reopen the portal and reunite with their friend, but not before making some new friends at Monster High and planning future collaborations.
Venom could be a formidable antagonist. He might be attracted to the chaos and different powers at Monster High. He could start infecting some of the students, turning them into his minions. Additionally, a group of anti - monster humans from Peter's world could find out about Monster High and decide to invade it. They see the monsters as a threat and want to 'cleanse' the school, which would put Peter in a difficult position as he has to protect his new friends from his own kind.
Well, perhaps there's a science experiment gone wrong at Monster High that creates a rift in the multiverse, pulling Peter Parker into their world. He teams up with Frankie Stein, who is also interested in science. Together, they try to find a way to close the rift while also dealing with the social drama at Monster High. The vampires might be causing trouble because they don't trust an outsider like Peter, but in the end, he wins them over with his heroic deeds.
A cool plot might be that Voldemort somehow discovers the existence of Monster High and plans to use the unique powers of the monsters there for his own evil purposes. So, Dumbledore sends a group of Hogwarts students, including Harry, to Monster High to warn them and form an alliance. The Monster High students, led by Frankie Stein and Draculaura, join forces with the Hogwarts students. They then have to use a combination of magic and monster abilities to stop Voldemort's plan, which could involve some sort of dark spell that would not only affect the wizarding world but also Monster High if successful.
Ben and Frankie Stein could be a really interesting pairing. Ben, with his ability to transform into various aliens, is always ready for action and adventure. Frankie, on the other hand, is intelligent and curious about the world around her. They could work together to solve problems that arise from the crossover. Frankie might be able to analyze the Omnitrix technology, while Ben could protect her from any immediate danger using his alien forms.
Frankie Stein and Raven Queen might get along well. Frankie is always curious and eager to learn new things, and Raven is an outcast in her own world who has a different view of the status quo. They could have deep conversations about what it means to be different and how to break free from the expectations placed on them.