As far as an 8th installment goes (and let me first be clear that I'm NOT pushing for this), never say "never". Even death isn't foolproof where this is concerned...
*Cough*Christopher Tolkien
*Cough*John Gregory Betancourt
*Cough* Not implying that either of the two situations above were contrary to the original author's (Tolkien and Zelazny, respectively, in the cases above)/estate's wishes, and I would imagine that a well-written will and an estate managed by loyal inheritors would slow such continuations down considerably...
Not to wander too far from the thread (he says as he continues wandering
)), but ckovacs would likely know whether Betancourt published his prequels with the Zelazny estate's blessing or not.....