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.
There could be. Fanfiction writers are known for their wild and creative crossovers. Given the popularity of Highschool DxD and the unique concept of Guyver, it's entirely possible that an Issei - Guyver fanfiction exists. You might want to check popular fanfiction websites like Archive of Our Own or FanFiction.net. Just search for relevant keywords like 'Highschool DxD Issei Guyver' and see if any relevant stories pop up.
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.