A possible storyline is that Naruto's yokai nature gives him a connection to the supernatural elements in the Highschool DxD world that others don't have. He might discover secrets about the world that no one else knows. For example, he could find a hidden place or power source that is related to both yokai and the devils or angels. His presence could also stir up old rivalries as some characters may see him as a threat to the existing power balance.
One possible storyline could be that Naruto, as a yokai, has to hide his true identity in the Highschool DxD world. He might get involved in the power struggles between different factions while also trying to fit in at school. Another could be that he uses his yokai powers to help out the main characters in Highschool DxD, like Issei, in battles against their enemies.
Given Naruto's status as a yokai, his interactions with the Highschool DxD characters would be complex. He could be a source of mystery to them. With the perverted Issei, Naruto might find his behavior a bit off - putting at first but would gradually understand his character. When it comes to the more powerful characters like Sirzechs, Naruto would show respect but also not be intimidated. He could also form a sort of rivalry with some of the more combat - oriented characters, like Kiba, as they compete to see who is stronger in different types of battles, whether it's a one - on - one fight or a group battle against a common enemy.
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.
Another storyline could involve Sage Naruto teaching some of the characters in Highschool DxD his ninja techniques. Imagine Naruto showing Issei how to use shadow clones to his advantage in battles or training sessions. This would not only add a new dimension to the battles in the DxD world but also create some really fun and comical moments as the characters from two different universes interact.
Well, one possible plotline could be Naruto, in the form of a hound, somehow gets transferred to the Highschool DxD world. He might have to deal with the different factions and powers there while still retaining some of his Naruto - like abilities.
One possible plotline could be Naruto being transported to the Highschool DxD world and having to adapt to its unique rules and social hierarchy. He might use his ninja skills to rise through the ranks in the school. Another could be that Naruto's presence disrupts the existing power balance in the DxD world, leading to various conflicts and alliances. Maybe the devils and angels in the DxD world are intrigued by Naruto's power and try to either recruit him or eliminate him.
Often, the storylines blend by having characters from Naruto end up in the Highschool DxD world. For example, Naruto might be summoned there by some sort of magic accident. Then, he has to navigate this new world full of devils, angels, and other supernatural beings while still having his ninja mindset and skills.
A fanfiction might explore a story where the Occult Research Club members are all transported to a different dimension. In this new world, they have to find a way back home while facing new enemies and challenges. Rias could use her leadership skills in a different way, and Kiba might discover new powers within himself to protect the group. This would also test the bonds between the characters in a whole new setting.
Maybe Venelana has some connection to a powerful artifact in the Naruto world. Naruto and Venelana then have to embark on a journey to find this artifact before it falls into the wrong hands. Along the way, they learn about each other's cultures and powers.
One possible storyline could be a tournament - based one. Naruto and Ophis could be entered into a high school tournament. Naruto would use his ninja techniques, and Ophis her dragon - like powers. Another storyline could involve a mystery in the school that they have to solve together. Naruto's detective skills (from his various missions in the Naruto world) and Ophis' knowledge of the supernatural could combine well.
One interesting storyline could be Fem Naruto joining the Occult Research Club in the Highschool DxD universe. She could use her ninja skills, like her shadow clone jutsu, in unique ways during club activities or battles against devils and fallen angels. Another could be her forming an unexpected friendship with Issei, perhaps teaching him some ninja stealth techniques which come in handy during his escapades.