You could write about a future where the three factions - devils, angels, and fallen angels - have formed a more unified society. However, there are still some elements within each group that oppose this unity. Issei could be in the middle of trying to maintain this peace while also dealing with a personal issue, like a new form of his Sacred Gear that he can't control. This would lead to a story full of political intrigue and personal growth for the characters.
One idea could be exploring new power-ups for the characters. For example, Issei could discover a hidden ability within his Boosted Gear that allows him to time - travel briefly during battles, adding a new strategic element. Another could be focusing on the relationships in the future. Maybe Rias and Issei have a family and the story could revolve around their children's upbringing in the supernatural world.
Another plot could involve him forming an unlikely alliance with the main characters of Highschool DxD. He might use his unique perspective as an outsider to solve a problem that the in - universe characters have been struggling with. For example, if there is a conflict between different factions, he could come up with a creative solution due to his different way of thinking.
Well, perhaps in the future Issei discovers a new power within himself that no one has ever seen before. This power could be related to his Sacred Gear evolving in a completely unexpected way. He then has to figure out how to control it while also dealing with the political intrigue in the devil world. There could be devils who are jealous of his newfound power and try to undermine him.
A more complex plot could be centered around a prophecy that foretells the destruction of the Highschool DxD world. The characters then have to embark on a journey to find the source of this prophecy and try to prevent it. Along the way, they discover secrets about their own powers and the true nature of the different factions, and have to make difficult choices that could either save or doom their world.
A third plot idea might be Sona trying to modernize the traditions of the devil world within the context of the school. She could face opposition from more conservative devils, and the story could explore her struggle to bring about change while still maintaining her position and respect.
One idea could be a new character who is a killer but has a hidden noble purpose. Maybe they are targeting rogue devils that are harming humans secretly. This would add an element of mystery and action to the fanfiction.
Maybe Jesus could have a special mission in the Highschool DxD universe. For example, he might be there to stop a great evil that is brewing and uses his powers in a way that is different from the traditional powers in the series. He could team up with Issei and the others, bringing a new dynamic to their group.
One interesting plot idea could be Murayama discovers a hidden power within herself. Maybe she has a connection to an ancient artifact in the Highschool DxD universe that gives her unique abilities, like the power to control time briefly. This could lead to her having to protect this secret from both the devils and the angels in the story.
A possible plot is that a group of students in High School DxD start a gaming club, but they accidentally discover that the games they play are linked to some ancient and powerful magic. As they delve deeper into these games, they uncover secrets about their world and have to face off against enemies who want to use this magic for evil purposes.
One idea could be a new character who is a futa and has unique powers related to both male and female aspects. They could join the Gremory group and cause some interesting dynamics, like having a different approach to battles and relationships within the group.
One idea could be exploring Kuroka's past more deeply in the High School DxD universe. Maybe she has some hidden relationships or secrets from her time before joining the main story's events.