I recall a scene in The Mists of Avalon where Morgaine balks at the suggestion that one day she might do a specific act. I don't recall now exactly how it was worded, or what it was (and whether it was killing or betraying someone). Her mentor said, "do not name the well from which you will not drink." And it turned out that, many years later, she had to do the thing that she had considered to be so heinous because she was put in a position where if she didn't many people would die.
Likewise (but much less seriously!) sometimes you have an author who states that the book they've just written is a one-off story and they won't write a sequel. Or the author of the best-selling, smash hit, movie-spawning series that defined a generation states that the story arc is finished, that they won't write another story about Stevie McStevenson, because Stevie McStevenson is done at the end of Stevie McStevenson Saves the World. But then that first author says, "well, y'know, I think I might have another story about this character." And the second author announces that, "whattaya know, I'm releasing another story about Stevie McStevenson that takes place several years later."
And a lot of people's reactions on an author going back on their "I'm not writing another story about these people" statement is to accuse them of selling out, of doing it for the money. And I'm sure there are cases where that is exactly what happened (and you can usually tell, because the sequel is sub-par). But there are also some, I'm sure, where the author who has publicly announced that there is nothing more to say, suddenly has a revelation while driving, or in the shower, or any of those other random moments where inspiration comes, and they say, "Oh, my god - there IS another story to tell!"
Why do I bring this up? I'm currently working on my oldest partial novel. I started it in November of 2009 with the very definite "knowledge" that it was one story. "Only ONE novel-length story will be told about Reyala," I told myself. And for a long time I actually worried that this might be a detriment when I go to pitch The Wolf and the Sheath to a potential publisher, as it's pretty clearly Young Adult, but Young Adult series are the money-makers that publishers want. "But, whatever," I thought, "if the story's good enough, someone will take a chance on it."
But a month or two ago, an idea came to me. Just a little idea in the shower. Sure, Reyala's story arc is done at the end of Wolf and Sheath, but what about her brothers? And just like pushing a pebble down a snowy hill in a cartoon, this idea picked up speed and, lo and behold, I have a basic plot outline for not one but two "sequels."
I use sequels in quotes because they are not continuations of Reyala's story. Reyala is the main character in The Wolf and the Sheath, but the ideas I have for the next stories focus on each of her younger brothers in turn. She would be in each of them, but she's not the central focus. Plus there may be a few years between W & S and the next sequel, and there would definitely be several years between the second and third story. So maybe they're not sequels so much as companion pieces. But, again, for 8 years I thought, pretty strongly, that there was only 1 story to tell here and now... we'll see if I do stop at 3.