In some cases, it could be a lack of understanding of appropriate boundaries. They don't see the harm in taking something that is loved by many, like a popular movie or book, and twisting it into something that is sexual and abnormal. They might not have a proper sense of what is acceptable in the realm of storytelling.
Some people get ideas for really weird fan fiction when they're in a really creative or dreamy state of mind. Maybe they had a really strange dream about the characters and then decide to write it down as a fan fiction. Or they could be brainstorming with other fans and one person throws out a really wild idea, and they all run with it. It's also possible that they just have a very unique sense of humor and want to create something that will make other fans laugh or be completely shocked by the strangeness.
One of the really strange ones might be those that involve odd character pairings from completely different universes. For example, pairing a cartoon character with a historical figure in an erotic context. It's just so out of place and goes against the normal perception of these characters.
Authors of science fiction often get their ideas from a combination of things. They could be inspired by dreams or nightmares. Some might have a fascination with the unknown, such as what lies beyond our galaxy. They think about the 'what ifs'. What if humans could communicate telepathically? What if there was a planet where gravity worked differently? Also, they may use their own life experiences. If they have traveled a lot, they might write about interstellar travel. If they've had experiences with technology going wrong, they could create a story about a world where technology has turned against humanity.
For me, it often comes from combining different elements. Maybe taking a historical event and adding a fantasy twist, or imagining what would happen if two unlikely characters met. Sometimes, random thoughts or dreams can turn into great story concepts too.
Read a lot. Different books can inspire you with various concepts and themes.
One way is to draw inspiration from your own life experiences and twist them into fictional scenarios. Maybe a memorable trip or a challenging relationship could be the basis for a great story.
Some authors for novels get ideas from their own experiences. For example, if they've had an interesting travel experience, they might base a story on it. Others might be inspired by historical events. They read about something that happened in the past and think about how to turn it into a fictional story.
Manga artists can come up with ideas by imagining alternate realities or futures. They might combine elements from various sources like history, mythology, or science fiction to create unique storylines. Sometimes, a simple conversation or a news article can spark their creativity.
One way is to explore some of the less - known fan fiction websites. These often have a wider variety of content, including the really strange ones. You can also search for tags like 'weird crossover' or 'unusual concept' on popular fan fiction platforms. Another option is to join fan fiction communities and ask the members for their recommendations of the weirdest fan fictions they've come across.
Read a lot of different types of literature. It can spark your creativity. Also, try imagining 'what if' scenarios and build a story around them for your flash fiction.