I was thinking the same lines as regards profit and non-profit legalities. Still, the author or late author's family may object to certain fan-work ideas which would affect the setting or characters. I have wondered what Steve Jackson would have said of The Curse of Drumer (although I don't think he's ever read or likely to read it); I'd appreciate his opinions and criticisms since it is a sequel to 'House of Hell'.
While we're discussing Steve Jackson: has a fan written a gamebook prequel or sequel to 'Creature of Havoc'? This was my very first FF gamebook - yet perhaps not the best choice for a beginner due to its difficulty rating. But I will say that, over the years, I've become to appreciate the intriguing history, fascinating characters, the complexity and amount of effort put into the gamebook.