Stumbling across a seed for fanfiction feels a lot like finding a stray bookmark in an old book — suddenly a story wants to be told.
I love hunting prompts in places that aren't strictly fanfiction hubs. For example, I'll scroll through random Wikipedia articles until a quirky historical fact sparks a what-if: what if a minor inventor from the 1800s got pulled into 'Steins;Gate' or what if a background noble from 'Pride and Prejudice' had secretly been a spy? Song lyrics and album concepts are another rich vein — a melancholic chorus can suggest an emotional arc, while a concept album can be the scaffolding for an AU. I also browse the 'Writing Prompts' subreddit and threads on Tumblr, where a single line like "your mentor betrays you on the day of your coronation" blossoms into scenes.
If I want something visual, I flip through fan art on Pinterest or DeviantArt and imagine the moment before or after the image. Games and lore wikis are gold too: a throwaway codex entry in 'The Witcher' or a side quest in 'The Legend of Zelda' can become the heart of a short story. I usually jot a few beats, pick a POV, and add a constraint — like writing only in letters — and it becomes a little world I can't wait to play in. I always end up more excited than when I started.