So I've had another idea. I think it's a good one, too. At least, I feel like it could be. Obviously I don't know until I start it!
At the moment it doesn't seem to have a defined plot, but I have four solid characters, all of whom I am seeing pretty clearly. I know my plot will develop out of what they decide to do, and I do have a vague idea of how the book will start and how it will finish, so I guess it's a case of letting them tell their stories.
Now if only I had more time and energy to start it!