Fan fiction writers use existing characters and settings from works of fiction they like, books, films, comics and so forth, and create their own story with them. Sometimes closer to the original work, sometimes wildly inventive.
For example, someone likes the film ‘Dead Man’ and decides to write an episode that might fit in somewhere in the opening of the story line. The result is the story
How the West West Was Won, and Where It Got Us
by Nightspore
Story Codes: M/m, nc
Synopsis: William Blake, the accountant from Cleveland, on his way to the town of Machine, California.
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Or, someone is a fan of the TV show Xena, grabs the two main characters, and dumps them into a new setting to let their timeless love story unfold.
Shadows of the Soul
by Melissa Good
Story Codes: F/f, romantic and mushy, violent (of course)
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Not all fan fiction has erotic content, or kinky content, but the temptation sure is there and often yielded to with great enthusiasm and creativity. Or, as the editors of another fan fiction archive put it: Certain fictional characters scream, ‘Tie me up! Please!’
A number of people start out writing fan fiction, Pauline Réage fanfic for example, and then move on to their own characters and settings. As far as I can see... as you, Mad Lews, have no trouble creating original characters... there’s no problem maintaining your very own fan base!