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.
Naruto would probably be very friendly and curious. He'd be excited to meet all the different types of beings in the Highschool DxD world. He might start by trying to make friends with Issei, sharing his stories about his own battles and adventures in his world.
One idea could be that Naruto is somehow transported to the world of Highschool DxD. He could use his ninja skills and chakra in the new world, maybe even teach some of the characters there a thing or two about combat. For example, he could show Issei how to better control his power using chakra control techniques.
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.