The plot could be affected in a number of ways. If the character gets inspired by the Naruto fanfiction, they might start to train differently. For example, if they see how Naruto characters train hard to master their skills, they could apply the same dedication in Highschool DxD's setting, which might lead to them achieving new powers faster and changing the power balance in the story.
Well, it could be really interesting. The character from Highschool DxD might find the concepts in Naruto fanfiction very strange or inspiring. For example, if Issei from Highschool DxD reads Naruto fanfiction, he might be intrigued by the ninja techniques and different power systems. He could potentially even try to incorporate some of those ideas into his own battles or ways of thinking.
One interesting story could be where the main character discovers a mysterious watch that gives him unique powers within the Highschool DxD universe. This watch might allow him to control time in battles or gain insights into the hidden secrets of the different factions.
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.
You can try fanfiction websites like FanFiction.net or Archive of Our Own (AO3). They usually have a wide variety of fanfictions, including those related to Highschool DxD and the multiverse concept.
Well, in some fanfictions, they could travel to a multiverse where the rules of the Highschool DxD world are completely different. For example, the devils, angels, and fallen angels might have different hierarchies or abilities. This can lead to really exciting plot twists as the characters have to adapt to these new environments and find a way to get back to their own universe.
You can start by looking on popular fanfiction websites like Fanfiction.net or Archive of Our Own. Use the search function and type in 'Highschool DxD watches'. This will bring up a list of relevant stories.
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.