One common theme is loyalty. Pip is always loyal to Seras, and this is often explored in the fanfics. For example, in many stories, he will go to great lengths to protect her.
Another plot could be that the baby gets lost in a mysterious ninja realm. Sakura and Naruto then have to embark on a dangerous journey to find their baby. Along the way, they face numerous challenges and also learn more about each other's love for their child and for each other.
Check the plausibility of the story. Even if it has elements of truth, if the overall story seems too far - fetched, it could be a desperate lie. For instance, if someone says they were in a minor car accident but then claim that it led to them being in a coma for a week, when the damage to the car was minimal. This shows that they are taking a true event (the accident) and stretching it to an unbelievable extent.
Isolation. When little miss is all alone, like in the abandoned asylum or in the middle of the forest during a camping trip, it makes the situation more terrifying. There's no one to turn to for immediate help, and she has to face the fear by herself. For example, in the asylum story, she was surrounded by the spooky whispers and had no one to share her fear with.
One way is through proper media rating and classification systems. By clearly marking stories with such content, parents and viewers can make informed decisions. For example, movies are rated, and this helps to keep inappropriate content away from younger audiences.
A BTS scary fanfic might involve a time - travel element where they go back to a historical era filled with superstitions and dark magic. They have to deal with curses and threats that seem to be targeting them specifically, all while trying to get back to their own time.
I'm not sure specifically as there could be many different interpretations in such a fanfiction. It might involve a strange power dynamic where Harry is somehow in a subservient position to Draco, perhaps exploring themes of control, rivalry, or unexpected alliances in a very unorthodox way.
Some common themes in band fanfic are about the band members' struggles. This might include dealing with fame, like how they handle the pressure of being in the spotlight all the time. There can also be themes related to the evolution of the band's music style over time. Also, the theme of a 'new member joining the band' and how that shakes things up is quite popular.
The Powerpuff Girls Blossom story is mainly about Blossom, one of the Powerpuff Girls. Blossom is the leader among the three sisters. In the story, she uses her superpowers, like flight and super strength, to fight against various villains in Townsville. For example, she battles Mojo Jojo, a mad monkey scientist who is always creating chaos in the city. Blossom often comes up with smart plans to defeat the bad guys, relying on her intelligence which is as strong as her physical powers.