One possible fanfiction could be about a crossover where Naruto characters end up in the Highschool DxD universe. Maybe Naruto himself could attend the high school and use his ninja skills in the unique situations there. He might form new friendships or rivalries with the characters from Highschool DxD.
One idea could be to create an OC youkai who is a kitsune. The kitsune could have unique powers like illusion - making and fire manipulation. It could start as an enemy to the main characters in Highschool DxD but later become an ally due to shared goals, such as protecting the human world from a greater evil.
Well, adding Poison Ivy to a Highschool DxD fanfiction could be really interesting. She could bring her unique plant - controlling abilities to the story. Maybe she could be a transfer student with a mysterious past, and her powers could either help or create chaos in the already wild world of Highschool DxD.
One potential new protagonist could be Issei's other friends or comrades. Since Rias is being bashed in this type of fanfiction, they might get more screen time and development. For example, Kiba could step up and show his leadership skills and unique abilities more.
One characteristic is the detailed character design. The novels describe the characters in great detail, which helps the anime adaptation in terms of visual representation.
Well, the art style in Highschool DxD New Anime Visual Novels is quite appealing. It often has detailed character designs that are faithful to the anime. The storylines can be engaging, with lots of character development and some exciting plot twists.
One idea could be an OC who is a dragon slayer but has a unique power that sets them apart. For example, they can absorb the powers of the dragons they slay instead of just killing them. This could lead to all kinds of new storylines as they become more powerful and are hunted by both good and bad forces in the Highschool DxD universe.
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.