It's Over, Or Is It?
Evil has been vanquished once again. The band of mercenaries battled the impossibly large force and somehow emerged victorious. The starship captain successfully completed another difficult mission thanks to her intellect and the competence of her crew. So, what happens now? It's up to you where you want to go from here, and you can often make the choice over and over. Sometimes the decision is forcibly altered in some way. When the first five books of the Well World series was finished, the author was asked when the sixth would come out. He had chosen to end the story there and not continue so he said, tongue firmly in cheek, "Well, if I have an idea, and someone offers me (insert insane amount of money here), there might be another one." The story ends with, "You know when someone really does offer you that much, finding an idea gets a lot easier.”
There are many options you can take when you’ve finished your story. I'll go over six possibilities here. These are not the only options. Of course you do more than one at different times. You can always choose to do another one later, when you're more ready. So without further ado, the beginning of the end.
1) Don't write any more in that setting. This is how many short stories wrap up. When the story is done, that's it. The story is told, no more answers are needed. Then the author can move on to something else.
2) Add an epilogue when the main story is done. In a series or a particularly long novel, this is quite common. Once the action is over, the author wants to let the reader know what happened next. An epilogue lets the author answer any unanswered questions from the story. A famous example of this is in The Return of the King, where the Hobbits return home, and are not happy with what happened to the Shire. (The details of the events are spoilers that I will not go into.) Following that are the years when Frodo finishes his book, and finally the meeting at the docks when Frodo and Bilbo complete their stories together. Epilogues are also popular with David and Leigh Eddings.
3) A sequel can often be good. This is also popular. For precision's sake, let's be clear what a sequel is. If a story is set after the original with the same characters or characters connected to them somehow, it is definitely a sequel. A sequel must have a unique plot. The plot may be related to the previous story, but should be able to stand on its own. Sequels are set in the same world, but at a later time. As examples, the seven Harry Potter books are a single story, none of them are sequels of the others. The Lord of the Rings novels tell one story, but together they may be a sequel to The Hobbit. Then again, The Hobbit may be a prequel to The Lord of the Rings. It can get confusing. Michael Moorcock seemed to favor sequels despite his many worlds of adventure. Most of his Elric novels are sequels. Corum and Hawkmoon both started with a three book series, and then continued in a three book sequel. Terry Brooks wrote several sequels to his first Shanarra Trilogy.
4) Write a Prequel. Where a sequel takes place after the first story, a prequel takes place before the original events. A prequel normally has different characters from the original, but there are often some in common. While a prequel normally covers events mentioned in the original, it must have a compelling plot of its own. The Simarillion covers the complete history of Middle Earth, and is a prequel to The Lord of the Rings. It succeeds because, while it is historical and sets the stage, it has an entirely different main villain and collection of heroes. The history of the world of the Belgariad is told in both Belgarath the Sorcerer and Polgara the Sorceress from different points of view. The Rivan Codex, also related to The Belgariad, would not be a prequel, as it was written as a more academically sounding history of the world. It contains a story that was the seed of Belgarath the Sorcerer, and that part could be considered a prequel. First King of Shannara chronicles the events before The Sword of Shanarra. Of course, Star Wars had prequels too, but they shouldn't form your opinion.
5) Write new stories in the same setting. These stories should stand on their own and not rely on the reader having read the first story. It's probable that you're writing a sequel or prequel if that is not the case. The new stories can use some of the previous story's characters and settings, though, but the characters should be minor characters and not ones that drive the plot forward, unless they are doing so in a very different way than in the previous story. Sometimes an author wants to makea character with a smaller part the role of protagonist. The Adventures of Tom Bombadil focuses on relatively minor players in the War of the Rings.
6) Move on to something new. A single world is too limiting for many authors. Chalker wrote stories of The Well World, the Changewinds, The Soul Rider and more. The Shanarra stories are complimented by stories set in the modern day and in a parallel magical universe, all written by Terry Brooks. The Eddingses wrote The Belgariad and The Malorean in one world, The Elenium and The Tamuli in another, and The Redemption of Althalus in a third, and then did others in other times and places. Just because you become famous for writing in one setting doesn't mean you can't branch out. Pseudonyms are popular with many authors moving into genres they are not well known for.
So, that's a whirlwind tour of the choices you have when you finish your story. All are good, but only if the author likes them. The only one I would personally not recommend is the unwritten -1, stop writing altogether. I'd rather you didn't go this route? You would not be doing yourself or your readers any favors this way.