Perhaps an adult Ben 10 story could focus on his relationship with his family. He might be teaching his own kids (if he has any) about the importance of using power responsibly. He could share his adventures and the lessons he learned along the way, like how to control the different alien powers without being consumed by them.
Well, 'fanfiction' means stories written by fans. If it's related to 'ben', it might be about a character named Ben. But the 'q0' and 'adult' parts are a bit strange. It could be that it was some sort of code or mislabeling. Maybe it was supposed to be a normal Ben - related fanfiction but got misnamed or there was some error in the categorization. Without more information, it's really just speculation.
Well, it depends on which comic series you're referring to. In some, Ben might die in a heroic battle, while in others, it could be due to a tragic accident or a villain's scheme.
A unique idea could be that Ben, as an adult, discovers a new alien form that is much more powerful but also much harder to control. He has to isolate himself to master this new form so that he can use it to save the world from a new and powerful threat that no other hero can handle.
One popular theme could be alternate universes where the characters have different powers or relationships. For example, an AU where Ben has to deal with his powers in a more adult - like world full of political intrigue related to the Omnitrix. Another theme might be character growth and development into adulthood, like how Gwen would handle her magical powers as an adult in a more complex society. And also, there could be themes of love and relationships, perhaps exploring Ben's romantic relationships in a more mature way compared to the original series.