Hey y'all. My nitpickery of Shadow Hunters (part two of the Dark Templar Saga) is taking a while, so for now, here's a nice rant on the longevity of stories.
One of the important things for a writer to understand is how much longevity any given story has. Not all stories are created alike. This is one of those esoteric things that you have to be a writer with some experience (of writing, not necessarily publishing) to understand. When a person is young, or is simply someone who just likes reading, they may crave more of a story they've just finished, to the point where they're mildly mad at the writer for not having a sequel out. Some fans may even send letters over it.
Thing is, some stories just don't work out with sequels. This is especially true of intense, strange stories with lots of complicated plot twists. A huge limiter on those kinds of stories is that the writer may run out of steam, and not have the kind of plot twists for a sequel that would make it comparable to the original. And if the characters have settled in at the end, like gotten married or found a job that pulls them out of whatever circumstances that caused the story to happen, then it can be aggravating to pull the characters up and force them to go through more nonsense.
One well-known example of this is the Matrix. When the first movie had been out a while and I heard they were working on a sequel, I really wondered how they were going to pull off a sequel. After all, Agent Smith was dead (or so we thought), and he's such a great villain that they're really going to have to work hard to create someone interesting enough to make up for it. Besides that, the ending of the Matrix was perfect. It left us with an image in our minds of how Neo could communicate with people and tell them what the Matrix is, and do as best he could to free others. The only possible sequel that's logical to the original is to create a sort of war-like, occupation sort of scenario. That, however, would take the primarily intellectual bent of the Matrix and make it something else entirely.
The creators of the Matrix probably realized that. Which is why they tried to keep a philosophic tone on everything, and by doing so, introducing philosophy that betrays everything we liked about the Matrix. Is the oracle a cool mystic? No, she's a computer program. Is Morpheus a wise guide? Nope, he's an old, deluded fool that believes in fake prophecies. Is Neo the one? Nope, he's some moron being tricked by the computers. Why the writers thought any of that was a good idea is beyond me.
Watership Down is another story that really shouldn't have had a sequel. It's the story of several rabbits, who must band together in order to find a new hope and escape whatever doom that Fiver knows will destroy their home warren. These rabbits go through many struggles with other warrens, other animals, and humans, and the story ends with them finally having the home they always wanted.
This is too content of an ending. There's two directions that a sequel writer can go with here: to create a dramatic disaster that forces the bunnies to act in an interesting way, or to write about what happens to the warren next if it stays as is. There's trouble with both options. For one, the characters worked really hard to establish their home, so it will offend the reader if all that work goes to waste in the sequel. If the warren stays the same, then regular bunny adventures might not be interesting enough for another book. No matter which direction the author goes, the ending of Watership Down saw all of the main characters growing older, allowing them to leave more daunting adventures to the younger, unfamiliar rabbits.
Ultimately, Richard Adams did create a pseudo sequel to Watership Down. Clearly he understood the problems of making a sequel to his original story, and ended up just writing a series of short stories about the rabbits in their new home. And it was boring.
Other stories that can't really have proper sequels are mystery stories and romances. Mysteries can't have sequels because the mystery has to be solved. Mysteries exist to mentally wrestle with their reader, and the reader tries to guess the end before it happens. If the mystery isn't solved in one book or episode, it can irritate the reader and make them wonder why they bother. For example, Lost. That show had so many mysteries and secrets, but instead of actually solving any issues, the writers of Lost simply add more things that make the audience wonder. It got so convoluted that Lost fans often warn curious onlookers that it's all a waste of time.
Romances, likewise have to stop somewhere, because they also exist on an emotional level. They're there to be fantasies of true love, and if two people end up together, the romance reader doesn't want her couple yanked apart for tension's sake.
What mysteries and romances have in common is that they have built-in ways to get around their sequel limitations. Despite the fact that mysteries have to end somehow, a typical mystery trope is the main character who solves crimes. This means that there can be as many sequels as there are different mysteries for the main character to solve. Though I do remember reading that mystery writers Arthur Conan Doyle and G.K. Chesterton both got really tired of their main characters after a while.
Also, horror has this same advantage, of being able to take the main character or main monster type and put them in a new situation. It doesn't work for as long a period, however, as the same monster can only be scary so many times -- fear hinges at least somewhat on the unknown.
Romance have an unfair advantage in that their audience will never go away. It's one of the most typical genres in all of literature. So even if it can't always have sequels, the writer can just pick up new characters and show them falling in love. In fact, that's sort of what the audience demands.
One of the best ways to create a story with an endless amount of potential sequels is to not create a story, but a world. JRR Tolkien is the father of this concept; his Middle Earth was not merely a setpiece for a story, it was a world created from his studies of history, myth, and language. He created a world so deep and rich that if somehow Tolkien could live on earth forever, he could milk the Lord of the Rings universe for a century or more. Heck, he even inspired dozens of writers to create worlds of their own, like the Forgotten Realms series or Shannara books. Star Trek and Star Wars are also examples of creating worlds for stories to take place.
However much I might complain about the Starcraft official novels themselves, novels in general were made possible by the fact that a game can't really go in as much depth about as a book can. Starcraft's mission based gameplay means that there can only be as much storytelling as can fit in a mission briefing or within the mission itself. Any story element that isn't combat related is something left to the player's imagination. Not only does this give Blizzard ample opportunity to write official novels, it creates a wealth of fanfiction opportunities and debatable issues, giving the fanbase a lot they can do on their own to fill in gaps. Very smart, that.
Still, the things I mention aren't always readily available to writers. We're not all game programmers. Not all of our stories, perhaps none of them, in fact, relate to creating its own world (coming of age stories, for example), and is something a world-building writer would have over the character developer or plot generating types. Romance as a genre can be somewhat stereotypical, and anyone with a unique message to say probably won't be interested conforming to the expectations of the romance audience.
The story itself may have limitations, totally unrelated to any issues with the writer's inspiration. Very emotional stories can tax a reader, and the reader may be very happy to reach the end and know that the beloved characters turn out alright in the end. Or, if they don't, that at least the torment is over. Adding another tense, emotional sequel to a book like this can be like repeating an argument with a friend -- draining to the point of not wanting to stick around.
Also, the writer may simply die. I'm sort of against writers who are not the original take on a story and write a sequel to it. This works better in franchises like Star Trek and Star Wars, because those are not only created worlds, but ones created by multiple people. Credit George Lucas all you want, but his better movies had more outside input, and apparently the Star Wars novels have a huge fanbase.
If, however, a story is written by one person or has no world to go with it, other people will most likely mess it up. Writers often go into their worlds with a specific vision in mind and a particular passion for what they want to say or create. This is why Lord of the Rings has no sequels. Sure, it's inspired a ton of other things, but no one can add to the vision of Tolkien, or compare to the amount of linguistic/historical research that went into creating Middle Earth. That, and one can argue that Middle Earth is impossible to add to -- you can't write about its past without knowing all of the history that Tolkien himself put into it, and you can't write about its future because the elves are leaving and it's implied that the hobbits and dwarves will too. Middle Earth was written as a mythology for the real world, leading from a legendary past into today. Thus, that means the eventually dissolution of everything that made Middle Earth unique.
All this is to say that every plot, character, or world detail has a shelf life. Whether due to potentially overused trends (zombies, superheroes, etc), a particular plotline, or just the fact that you've worked all the characters into appropriate places, eventually all stories must end.
So where does that leave your story? What kind of staying power does yours have? A staying power is anything that keeps the story you've created worth reading should a sequel come out. You might have a character that everyone loves, and who they want to know more about. Perhaps you have a world of your own invention, that enables stories to happen with completely different characters.
Or maybe, just maybe your story has no staying power beyond a trilogy, or even just one book. And you know what? That's okay. A good book needs no sequel, and pointlessly continuing a storyline when you lack inspiration or fan desire is what led Hollywood to the bog of remakes and reimaginings that it is today.
So think to yourself a few questions.
1. Where do I want my characters to ultimately be?
This is more important for character based stories than world or plot based ones, but since all stories have characters, it's not a question anyone can ignore. Where is the destination for your characters? Are they going to end up with certain jobs in the end? What will be their penultimate emotional status? Who will be with them and who not?
Now, you may be thinking to yourself that you have an idea for where your main character will be at the end of a book, but you know for a fact that when you get around to writing a sequel, you will want to take that character in different places. That's perfectly fine. It's not important for you to have the character figured out from birth to death, but just for you to have a goal and know what your particular goal is.
2. Am I creating a world or not?
This is more important for science fiction, fantasies, and westerns -- genres set in worlds that the readers don't generally live in and so the world has to be clarified whether the writer wants to or not. In modern day coming of age, women's stories (Confessions of a Shophaholic, etc), and action thrillers, this isn't so important. It can go either way for romances or histories, depending on when they're set.
What I mean by "world" is this: a unique, personally crafted place with its own scientific, cultural, and population. While technically all stories have a world, it's not always a place that the author needs to take the reader on a tour, particularly when based on the real world.
Basically, you need to ask yourself if you're creating a setting that's deep and rich like Lord of the Rings, or if you're just trying to show the exploits of your characters as you write your story.
3. How much gas does my plot have?
Plot is always a vehicle. It brings characters places and explains details about them and the world on the way. It may slow down, but it never stops. Eventually, the plot won't be able to carry the characters to interesting situations anymore, and you'll either need a new plotline or to just end the story.
So basically, how far is your story going to go? Is your story about stopping a crime syndicate of some sort? Okay then, so how long will it take to do so? What will happen when that's over? Is the main character going to go on to new and better things, or worse things, or will he die? Essentially, think of the primary point of your story, as in what the character's goal is, then estimate how long it will take to get there.
Don't think too far ahead on this, or else you may get so bogged down in sequel details that you slack on finishing your current story. Just have some idea of how long your plot will last you before you have to change gears or end it all.
Okay, so that's that. Remember, it's much better to end your story the way you want and leave the readers wanting more, rather than writing dissatisfactory nonsense that takes away your readers' ability to imagine the future of your world for themselves.