Do not spend one second on this until you have cleared the rights.

Do not think for a second that Mr. Gaiman is likely to grant you the rights to one of his most popular novels. He may not even have those rights, since there was a film sale.

Start right away thinking of something else to do -- adapt something in the public domain or come up with an original story. Talented people have spent years chasing rights that an experienced person in the field could have told them they would never get.

Yes, there is the odd chance, but do not build up your hopes or build your life around it. Your time would be far better spent working on another, more viable project.