If someone wrote a book in which, say, Con didn't get together with Roger Richardson ... would people feel that that was inappropriate because it contradicted something that'd already been published?
Since I can't see Con and Roger as a couple at all, that wouldn't bother me in the least!
I'm not sure my thoughts are coherent, but for what they're worth:
I think there's a difference between writing a completely different, separate take on a term that someone has already filled in, which I can't see any problem with, and writing within that person's universe, using her characters and situations.
But where does one draw the line? If it's acceptable to write fanfic about EBD's works, why should it not be acceptable to write fanfic about a fill-in, providing all proper acknowledgement is made to the author? I don't know the answer, just throwing the question out there.
I think the distinction between what is fanfic and what isn't, is becoming increasingly blurred. Does it stop being fanfic if it's authorised and paid for?
What category do things like Fifty Shades
fic) or Jill Paton Walsh's Wimsey continuations fall into? Is it OK to write Fifty Shades
fic (I expect there is some) or to write an AU of the period JPW has covered, or to write fic using her characters and situations?
As I said, not sure my thoughts are coherent, I'm just throwing them out there.