Well, such a fanfic could be really interesting. It might involve a character who has the abilities or traits from both 'High School DxD' and 'Naruto' universes and is leading the yokai. Maybe this leader could use the ninjutsu from 'Naruto' while also having the devilish powers from 'High School DxD' to rule over the yokai. For example, they could use shadow clones to patrol the yokai territory and at the same time use some sort of demonic charm to keep the yokai in line.
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.
Naruto would probably be very friendly and curious. He'd be excited to meet all the different types of beings in the Highschool DxD world. He might start by trying to make friends with Issei, sharing his stories about his own battles and adventures in his world.
One idea could be a crossover where Naruto characters end up in the world of Highschool DxD. For example, Naruto himself could be transferred to the school in Highschool DxD. He might use his ninja skills in the battles against the devils and fallen angels there. His shadow clone jutsu could really surprise the characters in the Highschool DxD world.
When writing, be creative with the power combinations. Let Naruto's Juubi - enhanced abilities interact in unique ways with the sacred gears or magic in Highschool DxD. For the story's atmosphere, blend the more serious and battle - oriented tone of Naruto with the sometimes comical and fan - service - heavy nature of Highschool DxD. And don't forget to give the story a proper ending, whether it's a happy ending where everyone becomes friends or a more bittersweet conclusion.
First, you need to understand the characteristics of each universe. For Highschool DxD, know the different devil clans, their powers and hierarchies. In Naruto, study the ninja villages, jutsus and the chakra system. For Tiamat, research her origin and abilities. Then, create a plot that can bring these elements together. It could be a quest to save a world, or a tournament where characters from all three universes participate.
One possible fanfic could be about Ravel from Highschool DxD finding herself in the Naruto world. Maybe she could use her unique abilities, like her magic skills, to interact with the ninjas. For example, she might befriend Naruto and help him with a difficult mission using her magical powers to provide an unexpected advantage.
The settings are also very different. Naruto is a more traditional Japanese - inspired ninja world, while Highschool DxD is set in a modern - day high school with a lot of supernatural elements. Writers need to blend these settings well so that the story doesn't feel disjointed. For example, if a character from Naruto enters the Highschool DxD world, how do they react to modern technology and the different social norms?
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.