There could be an amazing fanfiction where Naruto somehow gets involved with the world of High School DxD and the concept of a Dragon God. Maybe Naruto's powers would be a great addition to the battles and power dynamics in the DxD universe. He could form unique alliances or rivalries with the characters from DxD.
Another idea is that Naruto is reborn in the High School DxD world as a half - dragon due to the influence of the Dragon God. He then has to navigate the complex social and power structures of the DxD world while also hiding his true identity at first. As he discovers more about his new powers and the world around him, he gradually becomes a key figure in the battles and politics of the DxD world.
Yes, there are many. You can find them on various fanfiction websites. Some might focus on crossovers between the two universes, like having characters from Dragon Ball attend the high school in DxD. Others could explore unique power combinations or relationships between the characters from both series.
One common theme is power - up. Combining the power - up mechanics from Dragon Ball with the devil - related power growth in High School DxD. Another theme is friendship, as characters from both series form new bonds.
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.
You can try some popular fanfiction websites like FanFiction.net or Archive of Our Own (AO3). They usually have a large collection of fanfictions including those related to Dragon Ball Z and High School DxD.
One possible plot could be that the characters from Dragon Ball Z end up in the High School DxD world and have to deal with the different power dynamics and social hierarchies there. For example, Goku could be intrigued by the way the devils and angels in High School DxD have their own unique powers and politics. He might try to learn from them while also showing off his Saiyan strength.