One idea I've had intermittently for years:
An Event happens. Some very large, affecting many, many, many people in the same way.
Years later, a number of characters who were all children at the time -- perhaps all the same age -- are involved in a plot where each of them unfolds how the Event was a formative influence in his life. Oddly enough, although they experienced it in the same way, each of them attributes a wildly different effect on his character.
I slipped one such subplot in a novel I'm working on. The heroine goes on a journey and meets several woman who knew of her mother's wedding and what happened there -- she ended up repudiated before three days were up -- and recounted how it meant their very different choices in life.
But the idea has not died on the page. It would, in theory, have many possible manifestations, but the muse has not come up with the concrete Event or any notion of the affected children.
I suspect I will remember it every time someone suggests working out a character by assigning a trait and working out how the character came to have it. 0:)