A plot could revolve around the human side of the story more. Maybe the humans in the fanfiction start to develop their own unique combat techniques against the devils and fallen angels. They could form a secret organization dedicated to protecting humanity, and this organization could have some internal conflicts as well as external ones with the supernatural beings.
One idea could be a new character being introduced who has a unique power that challenges the existing power dynamics in the High School DxD world. For example, a character with the ability to nullify devil powers temporarily. This would create a lot of interesting situations where the devils would have to find new ways to deal with threats.
Another interesting plot could be a time - travel aspect. A character goes back in time to a crucial point in the High School DxD history and changes something small, but it has a huge ripple effect on the present. This could lead to new alliances, new rivalries, and a completely different set of challenges for the main characters.
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.
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.
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.
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.
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.
One idea could be a comical situation where a character accidentally releases a fart during a tense battle in the High School DxD world, and it somehow disrupts the enemy's attack. For example, the smell could be so bad that it makes the opponents gag and lose their concentration.
One idea could be Leviathan using her unique powers in a new way to solve a mystery within the school. For example, she might use her water - based abilities to uncover hidden passages or secrets. Another could be a story where she forms an unexpected alliance with a character from a different faction to face a common threat.
A possible plot is that the OC is a misfit who doesn't fit into the typical categories in the High School DxD world. They might be a human with no obvious supernatural powers at first, but through their wits and determination, they manage to form alliances with different factions. They could use their knowledge of human technology and psychology to gain an edge, and along the way, they develop a unique power or ability that no one expected.