[chrispenycate]

There are three quite different problems (at least) addressed here; the multi-volume work, conceived as such, where the author must plough ahead to avoid the ire of fans, the additions of sequels to essentially finished works, frequently due to fan pressure, and the placing of unrelated stories in an already existing framework.

If we consider Marion Zimmer Bradley's "Darkover" series, there are a couple of multi-volume sequences, ideas too big to fit into one book. There are several continuing themes – the renunciate one in particular – which return with no particular continuity, and have some examples which are indisputably sequels, some written by fans rather than the original author. And there are some stories which are set on the planet because it was a convenient environment, and saved writing the descriptions all over again.

Much the same can be said of Anne McCaffrey's "Pern" or Mercedes Lackey's "Valdemar".

But note that each of the three was turning out other series, unrelated material, at the same time.

Pratchett's "Discworld" is very much a third case; although he is obliged to write humouristically, he's otherwise free to set any idea that might appeal to him in that framework, with no requirement for consistency, or continuity. You invent firearms, but later don't think they fit the atmosphere? Just ignore them in later books, nobody'll notice. Not like Niven's "Known space" where every gadget has to be checked as to why it can't solve the problem in a later volume.