Need help with english essay on Great Gatsby.?

I'm presenting an english essay tomorrow and our essay topic is about the symbols of the Great Gatsby. I only need three symbols and to explain them and I have all three but one of them seems to be lacking. The theme that we chose for the book was about dreams and failed dreams. One of the symbols was the Valley of Ashes. I said that it was a desolate place that was uninhabited and was different from the neighbours towns but I don't know how that relates to the theme. Can I have any suggestions of maybe a new symbol that could represent the theme or something that could strengthen the point on the Valley of Ashes.