Perhaps Issei discovers that his connection to Great Red gives him the ability to travel to different realms more easily. This could lead to him exploring new places, making new allies and enemies, and learning more about his true nature. It could also involve him having to protect these realms from threats that only he, as the son of Great Red, can face.
Issei could also be involved in a storyline where he has to make a difficult choice between different paths. Maybe he has to decide between following a more peaceful route or a more aggressive one to protect his friends and the ones he loves, which would add depth to his character in the fanfiction.
His relationship with Rias would likely become more complex. She might see him in a new light, both as a powerful being in his own right and also as someone with a great destiny. There could be some tension as she tries to figure out how to interact with him now.
Perhaps Issei discovers that there are exorcist families with ties to the DxD world in ways that no one suspected. He gets involved in their internal power struggles and has to choose sides while still maintaining his loyalty to his original friends. It could lead to some intense battles and moral dilemmas.
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.