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's chakra system would likely be very unique in the Highschool DxD setting. Since chakra is different from the magic or power systems in DxD, it could give Naruto an advantage. For example, his ability to use Shadow Clones could be used for reconnaissance in the DxD world. His chakra control would also be a major factor. He could use it to enhance his physical abilities like speed and strength, much like how characters in DxD use their own powers.
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.