I've written lego-stories before, but I never liked them as well as the ones that wind themselves out of another story. For me it's like altering the angle of light on something and seeing how the shadow changes and writing the new one. It's why I like writing fic most. Of course, once I do notice the new shadow, I often have to run to keep up as it gathers weight from all the story-shapes I've internalized over the years.