I think I've mentioned that my chief source of ideas is dreams. In particular, the dreams I have after I've woken up very early, tossed and turned for an hour, and then fallen back asleep provide great inspiration (these dreams tend to be of the lucid variety and have a clear story to them). And yes, I had another good one last night : )
I really like this idea although I only had a clear dream of a couple of scenes. It's relative briefness means I don't know much about the plot other than it centers around the murder of a girl. I know who the main character is and thought up a few others while ruminating on the dream, but there's still a lot missing.
Because I am a pantser by nature, I'm not going to let this hold me back. I'm going to start typing and find out what happens as it comes. This book definitely will have a mystery quality to it, but I'm not sure it will be a whodunit, so to speak. No, I'm not going to blurt who did it right away (I'm not even sure!), but a lot of it will be about how terrible events can haunt you (the bulk of the story will take place ten years after the murder--it's the inciting event, but not necessarily the main plot).
I want to go write it now : ). Like lightning, this idea has struck and it's catching fire. I can't wait to see where it goes.