A really interesting plot could be about a magic or power transfer between the two universes. For example, some of the Naruto characters gain the Sacred Gears from High School DxD and have to learn how to use them. At the same time, the DxD characters could be intrigued by the chakra system of Naruto and try to incorporate it into their own powers. This could lead to all sorts of character interactions and new story developments as they train together and figure out how to best use these new abilities. And there could also be a harem element where characters from both universes develop romantic interests in each other, adding another layer of complexity to the relationships.
One plot idea could be the characters from Naruto finding themselves in the world of High School DxD and having to adapt to the different rules and powers there. They might form alliances with the characters from DxD to face a common enemy.
One plot idea could be that Naruto, with his ninja skills, gets transferred to the High School DxD world. He might accidentally join the Occult Research Club and use his Rasengan to defend the club from various threats. His positive attitude and never - give - up spirit could inspire the DxD characters and also attract the girls in the harem, leading to some comical and action - packed situations.
One plot idea could be that Naruto is summoned to the High School DxD world by a powerful magic user who mistakes him for a powerful demon. Once there, Naruto has to prove that he is not a threat while also trying to find a way back home. Along the way, he befriends some of the students at the school and helps them with their own problems, like protecting the school from a sudden attack by a group of rogue devils.
A plot idea could be that some of the devils in High School DxD want to learn the secrets of chakra from Naruto. So they kidnap him to their world. Naruto then has to find a way to escape while also dealing with the political intrigue among the devils, angels, and fallen angels. Another idea is that Naruto's shadow clones could be mistaken for some new type of summoned creatures in the DxD world.
A possible plot for 'naruto high school harem fanfiction' could be that Naruto is given a special project in high school where he has to work with a group of girls. As they work together, the girls start to develop feelings for Naruto. There could be drama within the group as some girls are more forward in their feelings while others are shy. And Naruto, being his oblivious self at first, slowly starts to realize the situation and has to figure out how to handle all these new emotions and relationships.
Well, a cool crossover idea might be the sharing of powers. Imagine if some of the High School DxD characters could learn Naruto's shadow clone jutsu. It would be really useful in battles. Also, the idea of Naruto characters trying to understand the complex social hierarchy in DxD could be very entertaining. They could end up forming alliances in unexpected ways, like the ninjas teaming up with the devils to fight against a powerful demon from Naruto's world that has somehow entered the DxD realm.
A plot could involve Kaido being sent on a special mission by the Devil King to infiltrate a human school. During this, he starts to develop real friendships with the characters in High School DxD. His initial motives are questioned as he begins to care more about his new friends than his mission, creating a lot of internal conflict.
One idea could be a Dragonborn character arriving in the High School DxD world and having to adapt to the unique school and supernatural environment. They might use their dragonborn abilities to stand out in the Rating Games.
Well, in 'high school dxd torture fanfiction', a plot could involve a character being physically tortured in a magical prison where the pain is amplified by dark magic. This could lead to the character having to find inner strength and allies to escape. Also, perhaps a character is tortured by being isolated from their friends and loved ones, which is a form of emotional torture.
A plot could revolve around a mystery in Akatsuki High School. There could be a series of strange events happening, like students disappearing or strange symbols appearing. The DxD characters would then have to use their skills and knowledge to solve the mystery, which could lead to uncovering a dark secret about the school's past.
Another plot could involve a time - travel scenario. Maybe a character accidentally sends themselves and some of their friends back in time to when the first devils were created. They would have to navigate this ancient world while also trying not to change history too much, or else it could have dire consequences for their present.