Since I've never wanted to write a series, I may be the worst person on earth to speak to plotting one. Because I don't. Sure. Sure, I have a SFR series. And now, an Urban Fantasy series. Neither of them started out being more than a single book each. Maybe it's a mental flaw of some kind. I simply don't think in arcs longer than a single novel. Everything I have, everything I am at the time goes into that single story. I'm down with the advice to hold nothing back for a later book. So I don't.
And then my editor says, "Have you thought about making this a series?"
This is one of those times when the acting degree comes in handy. I can act like I'm not clutching the phone, listening to the whine of my pulse in my ears. "Let me work up a few ideas for you," I say before hanging up the phone and succumbing to the urge to pound my head against a nearby doorframe.
It helped to realize there are two kinds of series: ones that follow a single, recurring character, and series that follow different characters whose story arcs are thematically related. I'd seen a lot of series with a single recurring character and I'd seen them go (in my opinion) totally sideways. I had long ago sworn I wouldn't fall into that trap. I simply wouldn't write a recurring character series. If you've read Nightmare Ink, you know how swiftly my vow fell by the wayside. The first series, the SFR, follows the arc of a war. Each book has a new hero and heroine. So at least for two whole books, I kept my word. O_o
Here's what it comes down to for me.
1. Did I think ahead and acknowledge a WIP could be a series?
If YES: recurring character, internal arc possible
If NO: different characters, external arc only
You've heard me talk about internal versus external conflict before, though usually as it regards a single character in a single book. In this case, Internal versus external conflict dictates what kind of series I can plot. A series with a recurring character most likely has a through line (across the entire series) that is internal to that main character - some emotional question that is bigger than one book can encompass. From the single character series I've read that I thought succeeded, it's clear the author thought that question out ahead of time and even if he or she didn't plot the series, per se, the high points were identified ahead of time.
The single character series I think flew into the ground don't have an internal arc - the authors use external arcs and events to keep their star moving. (Throw more monsters at the hero!) The problem is that without an internal arc for the character to follow, as more and more books get added to a series like this, the author seems compelled to keep trying to top the last story. This inevitably leads to shark jumping. It's never a good thing.
That's why, to plot a series, I don't actually plot. I figure out what I've got first. Is there an internal conflict or question that can propel my heroine through more than one book? If not, are there potential heroes or heroines lurking in my cast of secondary characters? Is there an external conflict that could drag several different characters through their own personal versions of hell? Once those questions are answered, I decide how many books I want in the series - usually based on how many characters I have available. The SFR is slated to be five books long. The urban fantasy - well - no one knows. Yeah. It's okay. I'll stop hyperventilating in a minute. In every case, I have a general end point in mind - a vision of how I want things to turn out. That allows me to begin coloring in the story between what has to happen to get the story from point A to point Z.
Plotting the internally motivated series boils down to using story events to challenge the heroine's core emotional question or issue. Don't be nice. Come at it from every conceivable angle. Let her win some. Let her lose some. The great thing about a series is that not all of the books have to end well. It's okay for her to fail. There's always next book for her to regroup and come at her issues again in a new way.
Plotting the externally driven series is about staying true to the arc of the action and fitting heroes to the action who will be most challenged by what the segment of action they face. The cool character idea gets axed if he or she doesn't contribute something emotionally interesting to whatever is happening in your world. It isn't to say there's no emotional truth in a series of this kind. There had better be. You still have to provide a complete character arc for each of the novels within the series. You have to end each character's story at the end of their book, that ending just has to serve the overall external arc.
Either way, my unorganized, unscientific method for plotting anything is a series of messy journal entries and then plucking the most interesting tidbits into a series bible document. The whole process is amorphous and subject to change since I am a pantser, myself, and I *like* to be surprised while writing a story. It does mean that it's terribly difficult to convey to anyone else how to do something that I suspect may be terra incognita - you have to get in there and discover it all for yourself.