I've had a niggling doubt in my mind for some time. I originally intended my Jack & Bea Mystery series to be a trilogy. The first in the series is Ashgrove Park, set in East Sussex, 1918-1919, and the second book is On Jacaranda Street, set in Sydney in 1923-1924. The final book was to be Moon Silver Hill, set in Boston around 1926 or so (but then I changed the location to Marblehead).
As I was writing the second book, though, it felt a lot like this was The End. The opening of On Jacaranda Street picks up where Ashgrove Park leaves off - Jack and Bea return a stolen artifact to New Guinea. Dr. Miller and Madame de Clermont are in Sydney at the time because Madame is on a spiritualist's tour of Australia (Australia was in the grip of spiritualism after WWI). So, it was natural for Jack and Bea to visit their friends.
It felt forced to then move to a third book but . . . I already have my outline for book three and will simply make that a stand-alone book with a whole set of new characters. The novel was to be set on the Massachusetts coastline and would have involved a dodgy Senator, a lighthouse, and Prohibition liquor smuggling. But instead of having a supernatural theme, it feels more like a straight-out crime/mystery thriller. So, that is what will happen - Moon Silver Hill is nixed and will become a stand-alone novel with another title (or maybe the same title, who knows).
First, though, I intend to write a novel set in Sydney in the late 1920s, based on my Harold Chesterfield character from On Jacaranda Street. You can find out about Harold here, but he's a hard-nosed Sydney police officer, and the novel (yet to be titled) will have him chasing criminals from Sydney razor gangs. This was to be a trilogy, but at this point, it will be stand-alone.
Then, I am going to resuscitate a novel I've had on my mind since around 2003. I've done a ton of research for it, made detailed notes, but never put pen to paper. It will be set in Thailand (a place I've been to many times for work and holidays) and will span the early 1900s to around the 1970s. This will also be a stand-alone novel.
The fantasy genre lends itself very well to a series, but I'm not feeling it for my writing or my genre.