Series Length

So, yesterday I was chatting with my wife, and the subject of the length of various series came up when I was talking about ideas for after the Ancient Dreams trilogy. She was the one who recommended that I put my thoughts up here.

When I come up with stories, I come up with a trilogy or a single story. Oh, the trilogy might have plot hooks that could be used to launch a separate trilogy about the same characters, or relatively minor characters in a book, but each trilogy is intended to close...