Interesting thread. It makes me think about a random story idea that blossomed into a shared universe with a hundred-odd years of history behind it. But that's an even longer post that will go off on many a tangent.
So to the topic at hand:
That random story idea generated into two-hour pilot script (don't ask, I was obsessed with screenwriting at the time). I had a notebook full of potential story ideas and character arcs. When I gave up the TV idea and adapted it into novel form, I took those character arcs and lengthened the story. A lot. First draft clocked in...um, long. So I split it, added, subtracted, and after a few years of fiddling, I now have a standalone, and a ready sequel. In this case, the sequel lended itself immediately to the first story. The first book can standalone with a few dangling threads, but the sequel completes the characters' story.
In the series I am co-writing, we intended a trilogy (it's going to be a lot more). Again, the first book is a complete story, but it leaves so many doors open for future adventures.
For my latest WIP, I am trying so hard to keep that puppy as a single novel. Can she do it? Stay tuned....