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.
A more complex plot could be that the future brother - in - law is actually a reincarnation of a powerful being from the Highschool DxD lore. His memories start to resurface slowly, and this affects his relationships with his family and the DxD characters. He has to balance his normal human life with his newfound identity and the responsibilities that come with it.
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.
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 great plot might be that Grayfia's brother - in - law is actually a half - demon or half - angel who has been hidden away from the main DxD world until now. When he enters the highschool, his presence starts to disrupt the delicate balance between the different species. Some see him as a threat, while others think he could be a key to a new era. As he tries to figure out his own identity and powers, he also has to deal with the normal high school problems like making friends, dealing with bullies, and passing exams in this supernatural high school.
A plot could also focus on a new character who is a hybrid of the two universes. They have the powers of both a devil from High School DxD and a wizard from Harry Potter. They struggle to find their place in either world as they are too different from the norm in both, and they embark on a journey to create their own identity and purpose.
One idea could be a new character joining the school who has a unique supernatural ability that no one has seen before. This could disrupt the existing power dynamics and create all sorts of interesting interactions and challenges.
One idea could be to have a new character enter the world of Highschool DxD who challenges the existing power hierarchy and calls out the flaws in the main characters' actions. For example, this new character might see Issei's pervy ways as not just a comical trait but something that is disrespectful to the female characters and start a movement to change the way things are done in the school.
Well, perhaps a fanfiction where Griselda gets transported to a different dimension. In this new place, she has to adapt quickly and find a way back home. Along the way, she meets new allies and enemies. This would be a great way to explore her character in a different setting.
One idea could be a succubus character trying to resist her natural instincts in the high school setting of Highschool DxD. For example, she might struggle with not using her powers to manipulate the male characters around her.
One idea could be a new character joining the school who has a mysterious power that both the devils and angels are interested in. This new character could disrupt the existing power balance in the school.
One plot idea could be Tiamat adjusting to the modern world in the Highschool DxD universe. Maybe she has to deal with the different social norms and the unique magic systems there.