One interesting concept could be the unique powers that the son of Trihexa might possess. Given Trihexa's status in the 'Highschool DxD' universe, the offspring could have extraordinary demonic abilities that set them apart from other characters. Another concept might be how this character interacts with the existing factions like the Devils, Angels, and Fallen Angels. Their heritage could lead to both suspicion and attempts to befriend or control them.
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.
The female characters from Highschool DxD, like Rias, would be intrigued by the unique personalities of the Naruto characters. Rias might see potential in some of them to join her peerage. And the Naruto girls, like Sakura, would be a bit shocked at first by the more revealing outfits and flirty nature of some of the Highschool DxD characters. But over time, they would bond over things like protecting their friends and the school. The Naruto boys would respect the power of the Highschool DxD devils and might even try to incorporate some of their fighting strategies into their own ninja combat styles.
The Highschool DxD novel is mainly centered around Issei's growth. He becomes part of the Occult Research Club at school. Along with his friends and comrades, he faces various challenges and battles against different factions. There are elements of comedy, action, and some romantic sub - plots, especially as Issei has his own harem of female characters who are also involved in the supernatural events.