What the BEST IDEA You've ever had, but Never Pursued?

I just responded to a post on the r/Tucson page about a mural/ (We have a lot of them here in town.) I reminded myself how I had the idea to take great photos of all the murals in town and creat a coffee table book. Never did, and now I guess I never will...but what a great idea!

What I want to ask you, is what was your best idea that you never pursued? Do you think you ever will? Has someone else run with it?