The unique combination of the Highschool DxD universe and the Neko element. In the Highschool DxD world, there are already so many supernatural elements, and adding the Neko aspect brings in a new level of cuteness and mystery. It can lead to interesting character interactions, like how the regular characters in Highschool DxD would react to the Neko characters.
You can try fanfiction websites like FanFiction.net or Archive of Our Own. They usually have a large collection of Highschool DxD Neko fanfictions. Just use the search function on these sites and type in 'Highschool DxD Neko' to get relevant results.
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.
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.