Issei would be a popular choice. Since he is the main character in the original series, having him time travel would create many interesting storylines. He could use his knowledge of future events and his growth as a character to change things in the past.
One idea could be that Issei goes back in time to his first day at school and uses his future knowledge to avoid all the initial embarrassments. He could also try to prevent some of the early battles and form alliances earlier.
Soaring Soldier City was an important character in " Demon High DXD " who had the ability to stop time. According to the novel, this ability came from the mysterious organization he was in, the Hand of Time. This organization had the ability to control time and passed on their power to others so that they also had the ability to stop time.
The reason why Soaring Soldier City had the ability to stop time was because he had received the power from the Hand of Time. The speed and direction of time flow on his body had been changed, allowing him to stop time under certain circumstances. This ability was very important to Soaring Soldier because he could use it to solve many problems such as avoiding danger, stopping the enemy's movements, and so on.
The concept of time manipulation through chrono vision in DxD fanfics can also be related to character development. A character who initially misuses the power, perhaps by relying too much on it and not developing other skills, could face a setback. For instance, they might encounter an enemy who can disrupt their chrono vision. This would then force them to grow and adapt, learning to fight without relying solely on seeing into the future, which makes for an interesting character arc.
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.