One idea could be that Naruto, through some magical or interdimensional event, ends up in the world of Highschool DxD. He could use his ninja skills and kind heart to win over one of the female characters, like Rias. Maybe he helps her with some underworld threats using his shadow clone jutsu, and in the process, a romantic bond forms between them.
Perhaps Naruto's chakra is sensed by the characters in Highschool DxD as a new and powerful form of energy. A shy character like Koneko approaches Naruto to learn about it. As Naruto teaches her about chakra, they grow closer. Naruto's positive attitude towards her despite her being a nekoshou makes her develop romantic feelings. Meanwhile, Naruto starts to see her in a new light as well, leading to a sweet and unique romance story.
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.
A fanfiction could focus on a shy devil girl who has a secret crush on Issei. She tries to overcome her shyness and the social hierarchy within the devil community to confess her love to him. This could involve her learning about human emotions and relationships in the process.
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.
One plot idea could be that Naruto is somehow transported to the world of Highschool DxD and meets Ophis. Maybe he has to help the characters in Highschool DxD deal with a threat using his ninja skills, and in the process, he forms an alliance with Ophis.
Well, a plot could involve Naruto having a delinquent past in his own world which somehow affects how he behaves in the Highschool DxD universe. He might be constantly getting into fights with the various factions because he doesn't fully understand their rules. And his rasengan could be a super - powerful move that both amazes and scares the characters in the new world.
One idea could be a tournament - style plot. Characters from all three universes are invited to a grand tournament. Naruto's ninjas, the devils from Highschool DxD, and Gabriel's angelic forces compete against each other in various challenges. This would showcase the different fighting styles and powers of each group.