When thinking about Neil Gaiman, The Sandman is one of the works that springs immediately to mind. The epic comic series about the brooding king of dreams helped put Gaiman on the map as a comic creator. But if George R.R. Martin had accepted an idea Gaiman pitched to him many years ago, Morpheus might not have been the Sandman but a character in George R.R. Martin's Wild Cards universe.
MTV News interviewed Martin about his experience collecting the Wild Cards anthologies, and Martin revealed that there was one creator that he always regretted turning down:
There was another writer who came along about the same time and wanted to be in Wildcards and actually approached me at the San Diego Comic-Con with an idea for a Wild Cards character. And he was this skinny British kid who dressed all in black. But he didn't have much in the way of credits so I kind of blew him off. But things have changed since then and I would love to have Neil Gaiman back writing for Wild Cards and I do rather regret blowing him off there at that San Diego Comic-Con in 1986 or so.
Later on, they catch Gaiman on camera, who tells a similar story (clarifying that it was 1987) and reveals that he pitched a Wild Cards character who lived in dreams. Of course, he would use that same idea just a couple years later when he was asked to reinvent the Sandman character for DC Comics. So perhaps Martin did us all a favor by not letting Gaiman work that particular concept out of his system too soon.