One important aspect is to focus on the character development. In Naruto, he has grown from an outcast to a hero. In a crossover fanfiction, you could show how he adapts to the new environment of Highschool DxD. Maybe he starts off being distrusted by the devils and angels because of his unknown power source (the Kyuubi). Then, as he shows his true nature, they start to accept him. Another point is the plot. You could create a conflict that involves both the threats from the Naruto world (like rogue ninja or Akatsuki members) coming into the Highschool DxD world and the internal conflicts within the Highschool DxD world itself, such as power struggles between the different factions. This would keep the story exciting and full of action.
A potential plot twist could be that the Kyuubi has a hidden connection to an ancient power in the Highschool DxD universe. Maybe it was sealed away long ago by the devils or angels and Naruto's presence awakens it. Another twist could be that Naruto is initially thought to be a threat by the main characters of Highschool DxD, but it turns out he is the key to solving a major problem they are facing, like a powerful enemy that is immune to their usual forms of attack but is vulnerable to Naruto's jutsu.
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.
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.
The female characters from Highschool DxD, like Rias, would be intrigued by the unique personalities of the Naruto characters. Rias might see potential in some of them to join her peerage. And the Naruto girls, like Sakura, would be a bit shocked at first by the more revealing outfits and flirty nature of some of the Highschool DxD characters. But over time, they would bond over things like protecting their friends and the school. The Naruto boys would respect the power of the Highschool DxD devils and might even try to incorporate some of their fighting strategies into their own ninja combat styles.
A fanfiction idea could be that Lucifer from the Highschool DxD universe somehow ends up in the Naruto world. In the Naruto world, he could be seen as a powerful and mysterious figure. He might interact with the ninja clans and shinobi, and his powers could be both feared and respected. He could also have some kind of connection with the tailed beasts or the Sage of Six Paths. The story could revolve around how his presence disrupts or changes the balance of power in the Naruto world.
The appeal lies in the mash - up of different elements. It's like a buffet of cool concepts. You get the action - packed ninja world of Naruto, the unique sword powers of Zanpakuto, and the supernatural high school setting of Highschool DxD. It's a chance to see your favorite characters in new and unexpected situations.
A more complex fanfiction could be about a joint mission between the two universes. The leaders of both worlds realize that there is a common threat that only the combined forces of Highschool DxD and Naruto can defeat. So, they send their best characters, like Rias and Sasuke, to work together. This would involve a lot of character development as they learn about each other's cultures and powers.
One idea could be a crossover where Naruto characters are transported to the world of The Highschool of DxD. For example, Naruto himself could end up at the high school and his ninja skills would be quite a shock to the devils and other beings there. He might get involved in the Rating Games, using his shadow clones and other jutsu to his advantage.
Yes, there might be such fanfictions out there. Fanfiction often combines different fictional universes in creative ways. However, not all of them are well - known or of high quality. Some might be more focused on the unique interactions between the characters from Naruto and Highschool DxD, exploring how their different powers and personalities would mesh in an unexpected relationship.