A third plot idea might be that there is a magical artifact that is affecting the balance between angels and devils. Raynare, with her knowledge of the angelic side, and Asia, with her connection to the church and the devils through Issei, work together to either destroy or safeguard the artifact. This would bring a lot of drama as they face opposition from different factions who want to use the artifact for their own gain.
One interesting plot idea could be that Raynare, who has had a change of heart after her initial actions, tries to make amends to Asia. Maybe she teams up with Asia to protect the school from a new, common enemy. This would show their growth and development as characters.
Their relationship might develop if they both find themselves in a situation where they are outcasts among their respective groups. They could bond over their feelings of not fully belonging and then decide to create their own place in the High School DxD universe. For example, starting a club for those who want to bridge the gap between angels, devils and fallen angels.
One interesting fan fiction could be about a new character joining the school and forming unique relationships with the existing characters. For example, a transfer student from a different supernatural realm who has different powers and a different view of the world in the DxD universe.
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.