An important part of writing is knowing when to just stop. I wish I was kidding, but it’s unfortunately true. There’s nothing wrong with staying in your world. There’s nothing wrong with writing a sequel either. I happily encourage it, if it’s a world you love to write in, but don’t just write books because a publisher says so. When I say this I think of trilogy syndrome. Trilogies seem to be all the rage in the literary world, or just long series, however, the latter doesn’t have this problem.
Trilogy syndrome is when a book, usually the second, is dragged out with filler and fluff so as to force the story onto a third leg. Anyone who has read a few trilogies can tell when trilogy syndrome takes place, because much of the book in between will feel forced towards the later half. A story should be told in however many books it needs, and that is why I have a lot of respect for the two book sets (a duology?) that appear from time to time. There was nothing forced, they just wrote what needed to be written. Another method that I’m fond of is the L.E. Modesitt Jr. method (only because I can’t think of many authors who do this), which is to write books across history in your world, giving a character no more than two books of their own, and only one in a few cases.
Is trilogy syndrome avoidable? Well, yes. If it means writing two longer books, rather than weakening two of them, or even writing one longer book. What matters is that the story gets told the way it was meant to, not because trilogies are popular.
There isn’t a ton for me to say on the topic without dredging up examples that I don’t have time for, so, for just a moment, I want to look at my current work. It’s presently titled Earth Shaker. The character, Elyas, drags himself quite reluctantly to the battlefield to go after the woman he loves, one of the Iron City Knights, because he fought in the last war, and knows just how vicious the honorable Drae can be. Of course he’s been getting stronger over the years with the help and influence of the Earth Goddess Anset, and more so in the days to come. As such, she is preparing him for a task, to act as her Envoy and kill someone he knows. Of course his decision is something that can shake the world. (Edit: pun unintended)
However, my story is about Elyas throwing himself back onto the battlefield and growing in power for the sake of love, and learning just what he can do. It’s a set up to what will hopefully be the second and final book. I’m not stretching the story more than it has to, however, I recognize that for certain events to take place, time has to pass, and I won’t be able to resolve certain conflicts in what I’d like to end in a single book and still maintain the writing style it has up until now. I recognize the story though, and I can at least confidently say that I don’t want it to suffer trilogy syndrome, and won’t let it as long as it continues on the path I seem to have envisioned for it.