Sometimes you start a sketch with something in mind, then it turns into something else. Eventually you turn the page on your sketchbook and forget it entirely. Then, you happen across it again (after a day of drawing stuff that looks like crud) and begin to think it has possiblitlies. This is one of my more successful ways of dealing with artist's block. Go back and look at something I've abandoned and see if it might fly.
And it might.