Perhaps Naruto is mistaken for a powerful devil or an unknown being in the DxD universe. Gabriel, being curious, approaches him. They start off on a rocky relationship as Naruto is confused by this new world. But as they get to know each other, they find out that their powers can be combined in unique ways. For example, Naruto's shadow clone jutsu combined with Gabriel's holy powers could create some really cool and unexpected attacks against their foes.
One possible plot could be that Naruto is somehow transported to the Highschool DxD universe and meets Gabriel. They might team up to face a common threat that is endangering both the ninja world and the supernatural world of Highschool DxD. Naruto's unique skills as a ninja could be combined with Gabriel's angelic powers in an epic battle against evil forces.
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.
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 possible plot could be Naruto being somehow transported to the world of Highschool DxD. He could use his ninja skills and chakra in the battles against the devils and fallen angels in that world. Maybe he teams up with Issei and his group, and Naruto's unique jutsus like Rasengan and Shadow Clone Jutsu surprise everyone.
A plot could revolve around Naruto attending Highschool DxD as an exchange student. He would be the odd one out, with his orange jumpsuit and headband. The students at the school would be curious about his powers and his world. Naruto could end up in a love triangle between a devil girl and an angel girl, just like Issei often finds himself in romantic entanglements. He would have to navigate the complex social and power dynamics of the school while still staying true to his ninja way.
In a 'Naruto x Kuroka Highschool DxD' fanfiction, the plot could be that Naruto is on a mission to find a powerful artifact that has been lost in the DxD world. Kuroka, being a local with knowledge of the hidden places, offers to help him. Along the way, they discover that there is a conspiracy to use the artifact to take over the school. They must race against time to stop the villains. Naruto's determination and Kuroka's slyness work well together as they outwit the bad guys.
One interesting plot could be Naruto being summoned to the world of Highschool DxD. His unique ninja skills and chakra would be a huge shock to the devils, angels, and fallen angels in that world. He could end up forming an unlikely alliance with Issei, perhaps teaching him some of his ninja techniques in exchange for learning about the Sacred Gears. They could then face a common enemy together, like a powerful devil who wants to use Naruto's power for his own evil plans.
The fanfictions also allow for a deeper exploration of the characters' personalities. Issei's determination and Gabriel's grace can be further developed. Writers might give them new backstories or put them in situations that are not in the original anime. This gives readers a fresh perspective on the characters and their relationship, which is what makes these fanfictions so interesting.
A great plot could be a tournament - style story in the 'Naruto x Rias High School DxD' fanfiction. The tournament could be a mix of ninja and devil skills. Naruto enters representing his ninja heritage and Rias enters on behalf of the devils. Along the way, they have to face opponents from different factions. As they progress, they start to understand each other better and develop a deeper bond. Naruto's determination to win for the sake of his new friends in the DxD world and Rias's desire to prove the strength of the devils would drive the story forward, and their combined efforts could lead to an unexpected victory.
Well, perhaps Naruto is on a quest in his own world and stumbles upon a strange portal. When he goes through it, he finds himself in the presence of Gabriel and Hannah Annafellows. Naruto's sense of justice would likely lead him to offer his help when he discovers they are in trouble. Gabriel, with his own powers, and Hannah Annafellows, with her unique abilities, would team up with Naruto. They could then go on a journey to retrieve a powerful artifact that has been stolen and is being misused. The story could be filled with battles, Naruto showing off his ninja moves, and the three of them growing closer as friends.