Well, there are prequel novels that offer background on some of the characters and events. Also, there are companion novels that explore additional adventures and story arcs within the same world.
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.
The 'High School DxD' light novel is about Issei Hyoudou, a perverted high school student who gets killed on his first date. He is then revived as a devil and becomes a servant to Rias Gremory. The story involves his adventures in the supernatural world, battles against other devils, fallen angels and various mythological creatures, as well as his growth in power and his relationships with the female characters around him.
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.
One possible scenario could be that Naruto characters are somehow transported to the world of High School DxD. Naruto's ninjutsu skills would be quite a shock to the devils and angels in that world. For example, Sasuke's Sharingan could give him an edge in battles against the supernatural beings in High School DxD. And Naruto's never - give - up attitude might inspire some of the characters there to face their challenges more bravely.
High School DxD's story is considered good by many. It offers a unique blend of genres and has a decent amount of depth. The character interactions and world-building add to its appeal. But, as with any story, it depends on personal preferences.
No, it isn't. High School DxD is a diverse mix of genres. While there are romantic subplots, the focus is on the action, supernatural elements, and the characters' growth and battles.