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.
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.
Naruto's experience in dealing with tough opponents in his own world would help him in the zombie - filled Highschool of the Dead. He would be able to quickly analyze the situation, figure out the weaknesses of the zombies, and come up with effective plans. His friendship - first approach might also lead to some interesting relationships with the other characters in this new and dangerous world.
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 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.
Well, first of all, the writers often take the unique elements from each universe. From Naruto, it's the ninjutsu, kekkei genkai, and the concept of the ninja villages. From Highschool of the Dead, it's the zombie apocalypse and the school environment. They then find ways to make these elements work together. Maybe a character with a Sharingan from Naruto can use its powers to detect zombies from afar. Also, they might use the character relationships. A Naruto character could form a team with some Highschool of the Dead survivors to face the challenges together. And they might also play with the different values of the two universes. For example, the loyalty in Naruto and the survival instinct in Highschool of the Dead.
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 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.