If I had a series about, well, series, I’d make a few key points. Namely, there’s my old yarn about writing your story as having “series potential” instead of REQUIRING a three- or five- or nine-book contract to execute your idea properly. We’re not in the Harry Potter boom years anymore, nor are we deep in a recession, but the market is still risk-averse. And signing up a debut writer for one unknown book, let alone three unknown books, represents a potential opportunity, sure, but also a big potential loss for the house. Basically, you’re in a much stronger position if you write one amazing manuscript with “series potential” (a few threads left open and the suggestion of future adventures that could be exploited) and then have the publisher asking you for a sequel, than you would be if you were the one needing multiple books to get your story told.
Now, how do you leave those threads open in a way that keeps your sequel options open while letting your manuscript seem whole enough to stand alone? Ah, now this is a good question. First, I would recommend that no more than three threads be left open. And they should be subplot threads with maybe one main plot thread, not all main plot threads. Your job is to resolve most of those by the end of Book 1. If you’re ending on a cliff-hanger or you’re leaving the main plot undecided, you’re not paying attention to everything the current market is telling you about sequels.
If, however, you haven’t entirely resolved one character’s problem, and your protagonist is still wondering about a certain element of the subplot, and the ending feels buttoned-up but you’ve hinted at the potential that everything could go to hell in a handbasket at some point in the future, then you’re doing it right. Future threat that may or may not come to pass is compelling enough to use as a launching-off point for a sequel. Present threat that’s not resolved slaps your reader in the face after they’ve spent four hours reading your story with a, “Yeah, you’ll have to buy the next installment and find out.”
Another thing I’ve noticed in a few manuscript is that seemingly random details are planted that stick out like a sore thumb. They have little bearing on the story that we’ve been reading so far. What gives? Invariably, the writer admits that they are “seeds being planted for the sequel.” This balances on the razor’s edge between “smart” and “silly.”
Let’s say that you are definitely planning a sequel if only someone would give you one, so you’re sneaking things into the manuscript that will only make sense once you get to execute the second (third, fourth, fifth, etc.) parts of the story you’re envisioning. That’s fine. To a point. But if the last third of your book starts to read like the prologue to Book 2, you’re in trouble with the reader. “Why are we spending so much time talking about something that has no precedent in the entire story I’ve just read?” they’ll wonder.
Balance is key to most things, in life and in fiction. Plant some details, leave some threads. But stick to your principles and your duties to the reader. Finish up the story you’ve invited them to read. That is first and foremost. Once you have a really solid resolution, then you can plant a few seeds. If you never get to do that sequel, they will be nonsense at worst, and not many people will notice. But if you’ve gone overboard and every second page hints at something that has no bearing on the present denouement, you’ve overstepped your bounds.