Well, perhaps a plot where the Chimera concept from Highschool DxD is applied to Naruto characters. For example, some of Naruto's friends or foes start to develop Chimera - like abilities. And in the Highschool DxD part of the fanfiction, the characters there are intrigued by these new powers. They might try to study them or even use them for their own purposes. Naruto and his comrades would then have to deal with the interference from the Highschool DxD world while also figuring out what's happening to their own powers.
Naruto would be different in this fanfiction in many ways. Firstly, his appearance might change slightly to fit the more modern and different aesthetic of the Highschool DxD world. His clothing could be a mix of his ninja gear and the school uniform of Highschool DxD. His power would also have new applications. For example, his shadow clone jutsu could be used in creative ways in the battles and social situations in Highschool DxD. His character would grow as he learns about the new races and abilities in the Highschool DxD world, and he might start to see his own powers from a different perspective. He'd be more of a bridge between the two worlds, bringing his ninja values and adding them to the mix of the Highschool DxD universe.
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.
In a Sharingan Naruto Highschool DxD fanfiction, the Sharingan could also play a role in terms of illusions. Imagine a scenario where the character uses Sharingan - based illusions to deceive the enemies from both worlds. They could make a Highschool DxD devil believe they are in a completely different environment like a dangerous Naruto - style forest filled with traps. And for Naruto characters, they could be made to see illusions of their worst nightmares or false threats from the Highschool DxD world, which would make for some really intense and engaging storylines.
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.