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.
The devils who oppose Grayfia's family could be the main antagonists. They might see Grayfia's brother - in - law as a weak link or a potential threat to their power. So they would try to get rid of him or manipulate him for their own gains.
Well, it could start with the discovery of the OC's identity as Grayfia's son. This would surely stir up a lot of reactions among the different factions in the Highschool DxD universe. He could be thrust into various adventures, like having to protect a certain character or retrieving a powerful artifact. His relationship with his mother, Grayfia, would also play a significant role, perhaps she has certain expectations of him that he struggles to meet while also dealing with his own desires and goals.
One common theme is loyalty. Grayfia is often portrayed as being extremely loyal in these fanfictions, whether it's to her superiors or to her new friends in the high school setting. Another theme is mystery, as her character often has secrets that are slowly uncovered throughout the story.
One common theme is her loyalty. Grayfia is known for her loyalty in the series, and fanfictions often explore this aspect further, like how she balances her loyalty to different characters or factions. Another theme is her power. Writers like to show her using her magic and skills in different situations.
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.
One possible plot could be that Naruto is summoned to the Highschool DxD world by some sort of magic accident. He then has to navigate the complex social and supernatural hierarchy of the school while also trying to find a way back home. Along the way, he might use his ninja skills to protect his new friends from various threats.
One key point is Grayfia's role as a powerful servant. She often has to balance her loyalty to Sirzechs and deal with the various chaotic situations in the Underworld. For example, she is involved in diplomatic affairs and power plays within the demon realm.
Well, it could be a story where the father - in - law has some special mission or task for the main character in the fanfiction. Since Highschool Dxd has a lot of supernatural elements, perhaps the father - in - law wants the character to retrieve a powerful artifact or protect a certain person. And this forms the basis of their relationship development throughout the fanfiction.
One way could be through a powerful magic - ninjutsu hybrid spell. Maybe a character with knowledge of both worlds accidentally creates a portal that merges the two. For example, if a shinobi in Naruto world somehow gets hold of a magic item from Highschool DxD and activates it in a wrong way.
Issei could be a potential ally. He is always getting into adventures and might see the OC as a valuable addition to his group. Also, Rias might be an ally as she is often involved in power plays and could use the OC's connection to Grayfia to her advantage while also providing protection to the OC in return.