One popular theme is 'what if' scenarios. For example, what if Harry had been sorted into Slytherin? Another common theme is exploring the backstories of minor characters. Many fanfictions focus on characters like Neville's parents and their time at Hogwarts. Also, the theme of alternate universes where the wizarding world has different rules or relationships is quite popular.
First, read a lot of the original Harry Potter books to really understand the world, characters, and magic system. Then, come up with an interesting concept. It could be a new adventure for Harry and his friends, or a different take on an existing event. Next, start writing and don't worry too much about grammar at first. Just get your ideas down.
One idea could be Sonya being a new student at Hogwarts who has a special ability to communicate with magical creatures. She teams up with Harry to solve a mystery involving some endangered magical beasts on the school grounds.
She could start as a shy and reserved character. As the story progresses, through her interactions with Harry and his friends, she could gain confidence. For example, if she participates in a Quidditch match as a substitute, she overcomes her fear of heights and becomes more outgoing.
Since I don't have detailed knowledge of 'fortune arterial visual novel', the main characters could be young people, like a male and a female lead. Maybe they have special abilities...
Sometimes, there are original characters in these fanfictions. For example, a merman elder who guides Danny in the underwater world. This character can introduce Danny to the rules and culture...
In the 'Chicken Little, sky is falling' story, the most prominent character is Chicken Little, of course. He is the one who sets off the whole chain of events when...
The cast of 'Bedtime Stories' is quite diverse. Adam Sandler is the central figure, and his acting is crucial to the movie's charm. The child actors in the cast are...
One key feature is the creation of an idealized society. In utopian short fiction, the setting often presents a world free from major problems like poverty, war, and inequality. For...
Basil is also quite popular. He has a certain charm and his relationship with other characters, especially his friendship with Evie, is something that fanfiction writers often expand on. They...