Well, with short story prompts for fiction, you should consider it as a springboard. If the prompt is about a time - traveling event, start imagining the character who experiences it. Is it a scientist? A regular person? Then, think about the rules of time travel in your story. How does it affect the character? What is the character's goal in this time - traveling journey? You can also add sub - plots related to the character's personal life before and after the time - travel.
First, find some interesting prompts. You can look for them on fanfic writing forums or websites dedicated to fanfic. Then, choose the fandom you want to write about. After that, start brainstorming how the prompt can fit into the world and characters of that fandom. For example, if the prompt is 'a character discovers a hidden power' and your fandom is Harry Potter, think about which character could discover this power and how it would change the story.
How about 'A young adult has to solve a mystery that has haunted their family for generations.' This could lead to all sorts of adventures and character development. There could be hidden clues, long-lost relatives, and unexpected twists.
You can look for inspiration in your daily life. Observe people, events, and emotions around you. Also, reading a wide variety of books and articles can spark ideas.