One popular theme could be the relationships between the characters. For example, exploring the romantic and sometimes more intense relationships between the main character and the various female characters in the series. Another theme might be the power dynamics within those relationships, as there are different levels of power among the characters in Highschool DxD.
The original Highschool DxD series focuses mainly on the story of the main character's journey in the supernatural world, his growth in power, and his relationships in a more general sense. However, the smut fanfiction often delves deeper into the sexual aspects of the relationships, which are not as prominently featured in the original.
The Highschool DxD manga is finished. It completed its journey, leaving behind a memorable story. Although it's over, its influence and popularity among manga lovers remain strong.
The Highschool DxD manga has indeed finished. It wrapped up after completing all the planned story arcs and providing a satisfying conclusion for the readers.
If Spawn were to interact with the main characters, he would bring a whole new level of seriousness. He has a dark past and a lot of baggage. When it comes to Issei's goal of becoming a harem king, Spawn might find it absurd but also interesting in a way. He could offer his own perspective on power and how to use it responsibly, which would be a contrast to the more carefree attitude of some of the Highschool DxD characters. And with the other characters like Akeno and Koneko, he'd be sizing them up, trying to figure out if they're threats or allies.
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.