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.
Maybe a new character could be the main character. A human who accidentally stumbles into the Highschool Dxd world and falls in love with one of the female characters there. Then he has to face the challenges of dealing with her father - in - law, who could be a powerful being like a devil king or a high - level angel.
The Highschool DxD manga has indeed finished. It wrapped up after completing all the planned story arcs and providing a satisfying conclusion for the readers.
If Spawn were to interact with the main characters, he would bring a whole new level of seriousness. He has a dark past and a lot of baggage. When it comes to Issei's goal of becoming a harem king, Spawn might find it absurd but also interesting in a way. He could offer his own perspective on power and how to use it responsibly, which would be a contrast to the more carefree attitude of some of the Highschool DxD characters. And with the other characters like Akeno and Koneko, he'd be sizing them up, trying to figure out if they're threats or allies.
The storylines can be combined by creating a common threat. Suppose an enemy emerges that is interested in both the power of the Bijuu and the power sources in Highschool DxD. This would force the characters from both universes to team up. Also, the cultural differences between the Naruto world and the Highschool DxD world can be explored. Naruto could introduce concepts like ninja clans and shinobi training to the DxD characters, while they could introduce him to the complex social hierarchies and supernatural politics in their world.
These fanfictions expand the Highschool DxD universe by introducing new characters like male Nekoshou. For example, they can create new storylines around their powers and how they interact with the existing characters in the DxD world. It might be a male Nekoshou with a special ability that changes the power dynamics in the school.