From a trope to a premise
Most fic ideas start as a trope or a single scene: rivals stuck somewhere small, or the conversation after the big betrayal that canon never showed. Type that, or press Surprise me for a premise written for the fanfiction shelf. The generator then asks up to three questions that make it specific, such as the setting, the point of view or the moment the two characters first clash.
More prompts if you want to browse
The prompts page on this site lists 36 ideas sorted by trope, from fake dating and fix-it fics to time loops and found family. Copy one into the box, swap in your characters and keep going. When the premise feels right, choose the heat and chapter length; the fic itself is written in the full editor on a free account, so the prompt you shaped here becomes chapter one.