When It Changed
It could be a relationship or a comedy routine; it could be the method of making widgets at the widget factory or a thousand-year-old cultural tradition. Whatever it is, it’s been that way for a long time.
Write a Ficly or series about the moment when something changes. The more seamlessly you work in your exposition and backstory, the better.
(Incidentally, “When It Changed” is a fantastic short story by Joanna Russ that you should all go read immediately. [grin])