I'm just coming to the end of my first novel, which would ideally be the first in a series. Despite my ideas for a bigger, sweeping storyline, I tried to provide as much closure as possible in my first book. My question is, how many loose threads are too many loose threads? It's a bit difficult to explain, but I guess I could liken my story to a James Bond film. In the first film, Bond defeats Dr. No, but we are given the hint that there is a larger threat looming out there (SPECTRE). I'm sure that if that had been the only Bond film, there would have been some disappointment, but on the other hand, the main villain is defeated and the hero gets the girl. In the same way, my book won't end on a cliffhanger, necessarily. The hero saves the day, the main villain is defeated, and there is closure for most major characters. However, subtle breadcrumbs are dropped that could/will lead to other conflicts. Does the sense of possible future plot complications render the book unmarketable to a publisher? Or when people say to make the first book a stand-alone, are they mainly warning against leaving the hero with a gun pointed at his head on the last page?