In Highschool DxD Volume 10, aside from the main group, there could be some important side characters as well. For instance, the leaders of other devil factions might make an appearance. They would bring new power dynamics to the story. Also, characters from Issei's school life, who are aware of the supernatural happenings to some extent, could play important roles in this volume, perhaps providing some comical relief or unexpected help during crucial situations.
Well, one key feature could be the development of the characters. In this volume, the relationships between the main characters might have evolved further. Another aspect could be the plot twists. It might introduce new challenges or enemies for the protagonist.
A great fanfiction could be a 'what - if' scenario. What if a certain event in Volume 10 had a different outcome? Maybe a character who was supposed to be defeated actually won, and the fanfiction could explore how that changes the entire dynamic of the story and the relationships between the characters.
I haven't read this volume specifically, but generally in the Highschool DxD series, it often involves the protagonist Issei's growth in power and his relationships with various characters like Rias. Maybe in this volume, there are new challenges for Issei in the supernatural world he's part of, and developments in his relationships with his friends and love interests.
You can start by checking out fanfiction websites like FanFiction.net or Archive of Our Own. Use the search function and type in 'Highschool DxD Volume 10' to get a list of relevant fanfictions. Then, look at the ratings and reviews to find the ones that are well - liked.
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.