Whenever I hear the word prequel, I think of Robert Jordan who--instead of writing the next much anticipated volume in his Wheel of Time series--took a year to go back and write a prequel, New Spring, and infuriated hundreds of readers. Not that I think Gaiman is going to do that. I know he has imagination to burn for any project he cares to take on. But since background and flashbacks are so frequently used to explain character motivations and such, I always wonder what authors hope to achieve by writing prequels.
No matter how clever and creative a writer might be, I still see an insurmountable problem in the fact that readers already know who will live and die because they've read the main series. An author might be able to spin a great story, but there won't be any of the dread and thrill that is possible in a sequel when a reader has no clue what's going to happen next.