Well, some characters might be amused. For example, Atsushi could be a bit shocked at first, seeing the different interpretations of Dazai in the fanfiction. He might find some of the more out - there scenarios quite strange but also interesting in a way.
One of the top fandoms for fanfiction is the Doctor Who fandom. With its time - traveling and space - exploring premise, the possibilities are endless. You can create new adventures for the Doctor and his companions, or introduce new alien species. The Supernatural fandom is also very popular. There are so many different mythologies and creatures in the show that you can base your fanfiction on. And then there's the Percy Jackson fandom. It combines Greek mythology with modern - day settings. You can write about new quests for demigods, or explore the relationships between different characters from different cabins at Camp Half - Blood.
No, it isn't. Fandoms vary greatly in their nature and the demographics of their fans. Some are more conservative or focused on a younger audience. In such cases, smutty fan fiction would be completely unacceptable. It can also create a divide within a fandom, with some fans being against this type of explicit content while others may be more accepting.
Yes, it can be common in some fandoms. In the Harry Potter fandom, for example, there are quite a few 'ron bashing fanfiction' pieces. Since the series has such a large and diverse fan base, different interpretations of characters are bound to happen. Some fans may focus on characters like Harry or Hermione more and see Ron as a hindrance in their relationships or adventures, so they write fanfics bashing him.
Dazai is not based on a true story. Usually, characters like Dazai are formed through the author's imagination and artistic expression to create an engaging narrative for the readers.