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.
These fanfictions expand the Highschool DxD universe by introducing new characters like male Nekoshou. For example, they can create new storylines around their powers and how they interact with the existing characters in the DxD world. It might be a male Nekoshou with a special ability that changes the power dynamics in the school.
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.
Yes, it is quite popular among fans of the 'High School DxD' series. The series already has a large following, and the visual novel format allows fans to engage more deeply with the story and characters.
An OC (Original Character) in a Highschool DxD fanfiction having the Rinnegan could be an extremely powerful and unique addition. The Rinnegan, from the Naruto universe, brings a whole set of abilities like the Six Paths powers. In the context of Highschool DxD, this OC could use these powers in various ways. For example, the ability to see chakra could be translated into seeing the magical energy in the DxD world. They could use the gravitational powers of the Rinnegan to manipulate the environment during battles against devils, angels, or fallen angels. It would make for a very interesting and overpowered character in the Highschool DxD fanfiction setting.
There is also 'Koneko's New Path'. This fanfic delves into a different scenario where Koneko has to face unique challenges that are not in the original plot. It's interesting as it gives a fresh perspective on her character. For example, she has to make difficult decisions about her loyalty to different factions, which really showcases her complex personality.