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.
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.
Well, the plot of Highschool DxD manga light novel is centered on the supernatural. Issei becomes a devil and gets involved with the devil society led by Rias. There are battles against other devils, as well as conflicts with angels and fallen angels. Each character has their own backstory and powers. For example, Rias has her own peerage which she uses in Rating Games to gain status. Issei's growth as a devil, both in terms of power and character, is a major part of the story. It's also full of comical moments, especially due to Issei's perverted nature which often gets him into trouble.
The Highschool DxD manga has gone a long way, presenting detailed plots and character developments. However, to know exactly how far it has gone, you might need to check the latest releases or related manga forums for the most up-to-date information.