Various famous books have had sequels written ( often by fine writers in their own right - I think Susan Hill wrote "Manderley" a sequel to Rebecca, and some attempted sequels to Jane Austen. Clearly there are also a lot of tacky rip offs of Tolkien, but I imagine the Tolkien Estate will protect all it's rights vigourously, and I would doubt that any "spin offs would be authorised, certainly while Christopher is alive. Part of the reason the Tolkien Estate officially distanced itself from the film was becasue it had no control over it ( although some members of the family were involved as individuals). In the long term who can say. Hookbill I think you are on dangerous ground ( but I am not going to kill you) to suggest that CRT added his own stuff to HoME. Obviously he has selected and edited, but it is quite clear what is his "editorial" and what is his Father's notes and if he has had to make a judgement call on a word these are annotated. It would be a fairer comment on the Sil to my mind actually where he had to make selections from so many versions and ( as he admits in HoME, may have made wrong choices eg with regard to Gilgalad's parentage) However the publication of so much of the drafts etc in HoME neutralises this to some extent. To some extent, given that CRT was so closly involved with his fathers work as it was being written, his speculation or subcreation would be fascinating.. yet he (as far as I recall ) restrains himself to documentary evidence a lot of the time. Although the odd anecdote (as with the origin of "Bingo" as the name first used for Frodo, slips out.
Usually CRT is accused of leaving stuff out rather than adding stuff
With all the Fan fic and RPG out there on the net ..I wonder if there would be a demand for published stuff? Any of us who indulge in these activities have our own theories how untold stories pan out - would we be satisfied with any other versions?