Issei is often still very popular in fanfiction. His growth as a character in the original makes him a great base for further development in fan - created stories. Writers can explore different aspects of his power and relationships.
Another concept is the exploration of different relationships. In highschool dxd system fanfiction, characters might form new alliances or romances that weren't as prominent in the official story. This can lead to some really engaging character development.
The magic system in Harry Potter could be integrated by having characters from Highschool DxD learn spells. For example, Issei could learn the Wingardium Leviosa spell to lift objects during battle, giving him an unexpected advantage. The Portkey concept could also be used to transport characters between the two worlds quickly.
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.