Although I have long enjoyed writing stories with connecting characters, in the beginning, I never set out to write a series. Both the Arizona and Oregon historical series began with a story I wanted to tell, with no idea it might lead to more. What led to more was always the feeling that there was more story.
An example of how this has worked is Arizona Sunset, which I wrote in the 1990s with this being the gist of the issues between the key protagonists:
Abigail sees herself as a spinster while Sam sees himself as an outlaw. She wants adventure and takes a chance to escape a life constrained by the mores of her time. When she falls into Sam's hands, he sees a chance for a real lady, a life such as he'd only seen from the outside. Neither understand that the other side has an unseen impact. Adventure means danger and often violence, which Abigail had never imagined. A home life is created with rules, the kind Sam has never followed.
Their relationship problems were the kinds of issues that go deep. Writing such stories always makes me think-- there is more here to explore. A seed is planted on what might be other angles to those kind of problems between lovers or would be lovers.
While I knew Arizona Sunset would be set in Tucson, the San Rafael Valley, Southern Arizona, Northern Mexico, and Tombstone, I had to decide what year would that adventure begin. Because the Earp brothers are still such a big deal for Tombstone, with diverse opinions as to what they were up to, I wanted them to have moved on. By 1883, they had. (All historical photos of old Tucson and taken at Arizona History Museum.)
1883 was an exciting time. Apache problems were possible. The region was still ruled often by the gun, while others were trying to establish a tony society with fine homes and parties. Arizona would never be given statehood if it couldn't show it was civilized. The railroad had gotten there in 1880. Change was on its way. Juxtapositions were everywhere, rustling in Mexico still supplied cattle for the big ranchers. Good guys and bad guys weren't always easy to tell apart (not sure they are today either, of course ;).
When I finally decided to release the book in December 2013, I saw it as standing alone. I had, however been thinking there were these characters in it, who seemed to have a story to tell. There was also much more also that interested me regarding those years in Tucson.
Priscilla, Abigail's best friend, is soft, sweet, rich, and part of the tony life Tucson wants to create. Cord, with secrets of his own, makes his living with a gun as a US Marshal. They are attracted to each other; but until Cord's estranged daughter comes to Tucson, there is no way they'd take a chance to see if they could find something more. Abigail and Sam show up in this book as well as another secondary character from the first one, who develops into a full-fledged villain. As my books often show, Martin, who had seemed mostly weak in the first book, is making choices that change his character-- not for the better.
There were things I researched about the government of Tucson, the US Marshal system (and how political it could be) and Arizona Territory at that time, which added to the story. Chinese and Yaqui cultures also added to the plot. Most people know how the Apaches influenced Arizona's history, but it's a state with a lot of diversity. Writing a romance uses what is real and brings to it what is imagination.
When I brought Tucson Moon (now, Arizona Moon) out in January 2014, I once again thought that was it. It wasn't; there would be six more in the Arizona series, with another swirling around in my head for a future book.
What I've learned about writing a series is let the characters tell you if they have a story worth telling. I've heard people ask how many books can be in a series. I'd say until the plots become forced, redundant or repetitive.
In my writing, I don't consider myself a plotter or a pantser but rather, like many authors, a little of both. I know generally where a book is going, but it's the discoveries along the way that make writing satisfying-- even the days I want to pull my hair out. While my Arizona and Oregon series all grew organically out of imagination and history, I've been more aware of the potential of secondary characters and even plan future series possibilities.