One idea could be that Naruto is banished to the world of Highschool DxD and his ninja skills make him a unique addition. He might use his shadow clone jutsu to attend multiple classes at once and make friends in a different way than the regular students. Another idea is that his chakra could be mistaken for some kind of special power in the DxD world, leading to both positive and negative attention from different factions.
In a 'Banished Naruto Highschool DxD Fanfiction', Naruto could be banished due to a false accusation in his own world. When he arrives in the Highschool DxD world, he starts to learn about their different power systems. He could end up training some of the DxD characters in his ninja ways, like teaching Issei how to better control his movements using ninja stealth techniques. This would create an interesting dynamic as they combine their powers for various challenges.
Perhaps a story where Kuroka transfers to the high school in the Highschool DxD setting and she meets Naruto who is also there for some reason. They could form an unlikely alliance against common enemies. Kuroka's cat - like abilities and Naruto's combat prowess would make for an interesting team - up. And they could also have some comical moments as Naruto tries to understand the strange rules and hierarchies of the Highschool DxD world.
One idea could be that Naruto characters attend the Highschool DxD academy. For example, Naruto himself could be a new student with his unique ninja skills. He might form an unlikely friendship with Issei. Another idea is a battle tournament where ninja techniques are pitted against the devilish powers in the DxD world.
Well, a plot could revolve around a mystery that involves both worlds. Maybe there is an ancient artifact that has the power to disrupt the balance between the Naruto universe and the Highschool DxD universe. Characters from both sides have to work together to find and protect this artifact. This would lead to a lot of character development as they learn about each other's cultures and values while on this quest.
One idea could be that Naruto is somehow transported to the world of Highschool DxD. He could use his ninja skills and chakra in the new world, maybe even teach some of the characters there a thing or two about combat. For example, he could show Issei how to better control his power using chakra control techniques.
One idea could be to focus on the characters' relationships outside of the main plot. For example, how the students interact during normal school days, like study groups or after - school clubs. Another idea is to create an alternate universe where the rules of the dxd world are slightly different in the high school setting, which could lead to new adventures and challenges for the characters.
One idea could be that Naruto somehow gets transported to the High School DxD world and has to deal with the devils and angels there while still having his ninja skills. He might use his shadow clone jutsu in unique ways during battles in the new world.
One idea could be a crossover where Naruto characters end up in the Highschool DxD world and have to adapt to the different rules regarding magic and devils. For example, Naruto could use his chakra in unique ways against the devils' powers.
Well, a cool plot could involve a character from Naruto getting transferred to the Highschool DxD universe. They could bring some ninja techniques and wisdom which would be really interesting in the context of the more modern and somewhat chaotic world of Highschool DxD. Or perhaps there's a mystery that only the skills of Naruto characters can solve in the Highschool DxD setting, like a strange demonic phenomenon that has ninja - like stealth and secrecy.
A plot could be that the Akatsuki from Naruto are after a powerful artifact in the Highschool DxD world that is related to Lucifer. This would bring the two universes into conflict as the devils and other supernatural beings in Highschool DxD would not want the Akatsuki to get their hands on it. Naruto characters like Sasuke or Kakashi could be pitted against the devils like Issei or Rias. And Lucifer could be in the middle, either trying to stop the Akatsuki for his own reasons or using them to gain more power in his own world.
Another idea is that a powerful artifact has been stolen and its pieces are scattered across the three worlds. Naruto, Gabriel, and the Highschool DxD characters must team up to find all the pieces. As they search, they encounter different challenges in each world. In Naruto's world, they face ninja clans who want the artifact for their own power. In Gabriel's world, they must deal with angelic politics. And in the Highschool DxD world, they have to fight off demons who also want the artifact.