The characters from Naruto can be integrated by having their powers be seen as a new type of power in the DxD world. For example, Naruto's Rasengan could be considered a powerful attack similar to a Sacred Gear's special ability. Sasuke's Sharingan could be something that the devils or other DxD characters are intrigued by and might try to either copy or study.
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 story where Naruto is somehow transported to the DxD universe and meets Ophis. He could form an alliance with her, perhaps because they both have a sense of being outsiders in the new world. Naruto's unique skills and his never - give - up attitude could influence the events in the DxD world in interesting ways.
One possible fanfic could be about an angel who gets trapped in the Naruto world and has to adapt to its ninja ways while still retaining some of the powers and characteristics from the DxD universe. For example, the angel might use its holy powers in a new way to fight against the akatsuki.
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.
One possible scenario in such a fanfic could be that Naruto is betrayed by his new comrades in the DxD world. Maybe they misunderstand his motives or are jealous of his power. For example, they might think he's using his abilities for his own gain rather than the common good.
I'm not sure if there is a specific fanfic exactly like that. There are a vast number of fanfics out there, and it might be a rather unique or niche combination. You could try searching on some major fanfic platforms like FanFiction.net or Archive of Our Own to see if such a story exists.
Well, Naruto has a wide range of powerful abilities like his shadow clones, rasengan, and chakra control. In the Highschool DxD universe which is filled with devils, angels, and fallen angels, his powers could be quite unique. His shadow clones could be used for various strategic purposes, like scouting or overwhelming opponents in battles. His rasengan, being a powerful concentrated attack, could be a real threat to the supernatural beings there. And his chakra control might give him an edge in adapting to different situations compared to the more traditional magic - based powers in DxD.
Character development is key. You need to show how Naruto would adapt to the DxD world and how he would interact with characters like Ophis. Maybe Naruto starts off being naive about the DxD world's politics but gradually learns. Also, use vivid descriptions. If Naruto is using his Rasengan in the DxD world, describe how it looks different or has different effects due to the different magical environment. And don't forget about pacing. Don't rush the story, but also don't make it drag on.
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.