Well, it could be that Naruto, as a Nephilim, enters the High School DxD world. He might use his ninja skills along with his new Nephilim powers. Maybe he has to deal with the devils and angels in the school setting, while also trying to keep his identity a secret. He could form new alliances and rivalries within the school, all while facing challenges that are unique to this crossover world.
One possible plot could start with Naruto being somehow transported to the High School DxD universe and being reborn as a Nephilim. As he starts at the school, he discovers his new powers are different from his ninja abilities. He could be caught in the middle of the power struggles between the different factions in the school, like the devils, angels, and fallen angels. His Naruto - like determination and will to protect could lead him to try and bring peace to the school, all while dealing with his own internal struggle of being a Nephilim in this strange new world.
I'm not entirely sure if there is a specific 'Naruto is a Nephilim High School DxD' fanfiction. Fanfiction writers are constantly coming up with new and creative ideas, so it's possible. High School DxD has its own unique universe with various supernatural elements, and Naruto has his own set of abilities and story. Combining them with the concept of a Nephilim can lead to a really cool and complex fanfiction. You could also try reaching out to fanfiction communities on social media platforms to see if anyone knows about such a story.
Perhaps in this fanfiction, Naruto and Kushina are reincarnated in the High School DxD world. They have to adapt to the new school life there. Naruto would surely stand out with his determination and fighting abilities. Kushina would be a motherly figure to some of the students who lack that kind of support. They both could change the balance of power between the different factions in the world.
One possible plot could be that Naruto, who is now half - youkai, gets transferred to the High School DxD world. He has to hide his true identity at first but then gets involved in the power struggles between the different factions. Maybe he uses his ninjutsu and new - found youkai powers to protect his friends and the school from a common enemy.
One possible plot could be that Kiba is made out to be a bully in the High School DxD setting. He could be picking on characters from the DxD world, which angers the main characters from that series. This then leads to a conflict where the DxD characters team up with some Naruto characters who also don't like Kiba's behavior.
One possible plot is that Gilgamesh views the High School DxD setting as beneath him at first. But as he gets to know the characters like Issei and Rias, he starts to see the value in their struggles. He could then use his vast resources and power to help them in their fight against stronger enemies, all while maintaining his haughty and kingly demeanor.
One possible interesting plot could be that Naruto, with his unique ninja skills, gets transferred to High School DxD's world. He might initially struggle to adapt to the different rules and magic there. For example, he could mistake a devil's magic for some kind of new jutsu. Then, he could end up teaming with Issei and his group to fight a common enemy that threatens both the ninja world and the world of High School DxD. Maybe an ancient evil that wants to use the power of both chakras and devil magic for its own dark purposes.
A great plot could be a tournament - style story in the 'Naruto x Rias High School DxD' fanfiction. The tournament could be a mix of ninja and devil skills. Naruto enters representing his ninja heritage and Rias enters on behalf of the devils. Along the way, they have to face opponents from different factions. As they progress, they start to understand each other better and develop a deeper bond. Naruto's determination to win for the sake of his new friends in the DxD world and Rias's desire to prove the strength of the devils would drive the story forward, and their combined efforts could lead to an unexpected victory.
One way could be through the power system. The EMS (Eternal Mangekyou Sharingan) from Naruto could be a unique power that a character in the High School DxD world acquires. Maybe a transfer student from a hidden ninja village arrives at the high school and has to keep their EMS a secret while dealing with the devils and angels in the school. This could lead to some interesting battles and interactions as the powers of the EMS, like the ability to use powerful jutsus, contrast with the magic and abilities of the High School DxD characters.
In a Naruto and High School DxD fanfiction, there could be a scenario where the protagonists of both stories meet in a neutral dimension. Issei from High School DxD might be intrigued by Naruto's ninja skills and determination. Naruto, in turn, could be curious about Issei's Sacred Gear. They could start training together, sharing their knowledge and skills, and ultimately have to face a common enemy that threatens both their worlds.
Firebrand could be a student in the High School DxD setting who has a strange connection to the Naruto world. Maybe they have dreams or visions of the Naruto world and start to develop ninja - like abilities in the High School DxD world. This could lead to them being an outcast at first, but as they grow in power, they become a key figure in protecting the school from threats that are both from the High School DxD universe and potentially threats that have some connection to the Naruto world.