One interesting concept could be the exploration of different character arcs in a death and rebirth scenario. For example, how a character's personality might change after experiencing death and coming back. Another could be the new powers or abilities they gain during this process, which can add a whole new dimension to the story.
Issei is often a very important character. His growth and development, especially in a death and rebirth context, can drive the story forward. Rias too, as her relationship with Issei and her role in the supernatural world of Highschool DxD can be greatly affected by such themes.
I'm not entirely sure specifically what 'dxd adult fanfic' is. 'DxD' might refer to a particular series (such as 'High School DxD'), and 'fanfic' stands for fan fiction. However, the 'adult' part implies it could be fan - created stories with mature or inappropriate content which is not suitable to discuss in detail.
The number of DxD novels can be a bit tricky to pin down. Different regions might have different releases and editions. Your best bet is to do a comprehensive search on popular book databases or consult with avid fans of the series.
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.
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