In fanfic these are often called "plotbunnies." For fic exchanges, there's often a prompts post where people list ideas for other people to write. I tend to throw plotbunnies around in clusters of half a dozen or so.
Random set, mostly erotica-intended but some are other genres:
- A polygamous Mormon family in the Territory of Utah has a secret: their husband wasn't born a man.
- A PI finds it's easier to get pictures of cheating wives/husbands if he pays a prostitute to seduce the person he's being paid to photograph.
- A married woman's teenage stepson is trying to seduce her.
- A sentient sword, an apprentice wizard, an ancient thief, and a lust potion—what could possibly go wrong?
- A new colony planet is having problems with fertility & take extreme measures to ensure survival of the species.
- Cabin boy is a girl in disguise, seeking her long-lost brother. He's now the captain, and once she's discovered, he'll need to find a way to punish her to avoid mutiny.
- Projecting telepaths are *everyone's* favorite sex toy.