"What if a serial killer videotaped his own murders and then sent that video to the cops?"
That's the first line I give for my novel Beholder's Eye. There's more to it, but essentially, when I first thought of this book, that's the what if question I came up with.
Now, this isn't the entire elevator pitch, but this gets me started. Try yours out on a group of people. If you are part of a writers' group, pitch them and see what they think.