One way could be through a crossover character. Imagine a character from Naruto's world ending up in the Highschool DxD universe with their Sharingan. They could use it in battles against the devils and fallen angels in the Highschool DxD world. For example, the Sharingan's ability to predict movements would give them an edge in combat.
One way to incorporate Naruto's Rinne Sharingan in a Highschool DxD fanfiction could be through a crossover event. Maybe a character from the Naruto universe accidentally ends up in the Highschool DxD world and has the Rinne Sharingan. Their unique powers could disrupt the balance in the new world, leading to various interesting plot developments. For example, they could use the Rinne Sharingan's abilities to see through illusions in the Highschool DxD world which is full of magic and deception.
A creative use could be using the Sharingan to copy the magic or abilities of the characters in Highschool DxD. Since the Sharingan can copy jutsu in Naruto, it could be extended to copy spells in the DxD world. This would make the character with the Sharingan very versatile in combat and magic usage.
A plot could revolve around the Sharingan OC being a target because their Sharingan is seen as a powerful tool. Devils might want to use it for their own gain, and angels might see it as a threat. The OC then has to fight to protect themselves and their friends. Also, the OC could use their Sharingan to unlock a hidden power within Issei, which changes the power balance in the DxD world. Another idea is that the OC comes from a parallel world where the Sharingan and DxD powers are combined, and they are on a mission to bring balance to the DxD world.
Well, for a Sharingan OC in Highschool DxD fanfiction, start with their appearance. They could have long black hair and a red and black outfit to match the Sharingan theme. Then, consider their relationships. They might befriend Issei and help him with his battles. Their Sharingan abilities could include things like predicting enemy moves accurately. Also, don't forget to give them some flaws. Maybe their Sharingan drains them of energy quickly, which adds an interesting dynamic to their character.
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.
If a Highschool DxD character got the Rinne Sharingan, they would gain a whole new set of abilities. They could potentially use the Rinnegan's ability to see chakra to detect the magic energy in their own world in a different way. For example, they might be able to see the flow of magic within spells and use that knowledge to counter them. Their combat abilities would also increase significantly as they could use the various powers associated with the Rinne Sharingan like the ability to summon creatures or use powerful elemental attacks.
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.
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.