The author him/herself acknowledges that there needs to be, or there is room for, further works/sequels.

Like DomA, I would say we should not make the attempt without extensive guidance from the original source - that's why The Silmarillion works.

On the other hand, I do believe that final authority for a reading lies with the reader, and if a work can be read convincingly as an organic continuation of a previous work then I don't see why not.

There's no "should", of course. It's more of a "wouldn't it be nice if...?"

For instance, Colfer's sixth Hitchhiker's novel feels to me very Adamsian in content, though not at all in style/prose. I can't syay it "should" have been written, but it's interesting to see it tried.

There's some very beautiful books out there, adding to the Sherlock Holmes canon - some written by Doyle's son, some by other people as continuations and additions to the originals. Others exist as homages, and very lovely they are too. (Okay, yes, some of them are pure shit and make me very nearly believe in book burning, but tolerance is a necessary quality of the reading life.)

(I'm reading The Independence of Miss Mary Bennett today, and it's great!)

Er. I think I'm at "it's a nice idea when it works".