Unfortunately, US authors generally are shy of fanwork collaborations, due to a minor controversy in 1980. https://en.wikipedia.org/wiki/Legal_issues_with_fan_

Unfortunately, US authors generally are shy of fanwork collaborations, due to a minor controversy in 1980. https://en.wikipedia.org/wiki/Legal_issues_with_fan_fiction#Authors https://en.wikipedia.org/wiki/Legal_issues_with_fan_fiction#... >Also noteworthy is the series of Darkover anthologies published by Marion Zimmer Bradley, beginning in 1980, consisting largely of fan fiction extended into her canon. These books led to a much talked about controversy. Bradley read something in a fan story that meshed well with a Darkover book she was currently writing, so she wrote the fan author, Jean Lamb, offering her "a sum and a dedication for all rights to the text." In a 1991 Usenet post, Jean continued, "I attempted at that point to _very politely_ negotiate a better deal. I was told that I had better take what I was offered, that much better authors than I had not been paid as much (we're talking a few hundred dollars here) and had gotten the same sort of 'credit' (this was in the summer of 1992)...a few months later I received a letter from Ms. Bradley's lawyer threatening me with a suit."[52] After Bradley's death, more information has come out supporting the fan's story.[53] >The rumor, however, was that Bradley had a skirmish with a fan who claimed authorship of a book identical to one Bradley had published and accused Bradley of "stealing" the idea, and the resultant lawsuit cost Bradley a book. Either way, her attorney advised her against reading fan fiction of her work. Versions of this incident are credited by many to have led to a "zero tolerance" policy on the part of a number of other professional authors, including Andre Norton, and David Weber. The only author/fan collaborative project I'm closely familiar with, Homestuck/Paradox Space/Homestuck 2.0, did not... entirely... dissolve into recriminations and lawsuits, but was not terribly profitable.