When writing a long-running series, there's a fine balance to strike between stagnation and pollution. By this I mean that a 30-book series, or a 7-season TV show, can go a bit stale if nothing changes from one episode to another. There's not a lot of world you can explore in 30 books that you can't in 20, at least without sacrificing some coherence. One the flip-side of this is allowing things to change too much, and to lose that thing which made them interesting in the first place.
I have two examples of this. 1) Dwarfs in Discworld, 2) Ferengi in Star Trek Deep Space 9.
The dwarfs have always had a strong 'racial' identity. Short, bearded, iron helmet, axes, apparent genderlessness. This makes them interesting, and TP has explored the mismatch between their culture and "ours" (or his version of it) to great effect. Part of this exploration took place through the development of Cheery into Cheri (i.e. an openly-female dwarf), and there were some funny scenes along the way (Cheri in ball-gown with sequinned axe in Fifth Elephant springs to mind). Alas, in Unseen Academicals we see the solo renegade become but a puff within winds of change blowing through mainstream dwarf society.
This, for me, killed the dwarfs as a point of interest.
Suddenly they aren't a unique race any more, but rather are morphing into versions of 'normal' humans. Suddenly, the differences which defined them and made them interesting are being discarded, and with it any dramatic interest in them.
DS9 made the same mistake with the Ferengi. For Cheri read Quark and his family. For sympathetic leader (Low King) read The Nagus. For open gender read loss of obsessive capitalism, and, yes, measures of gender equality (incidentally, for 'gold' read 'latinum' and the races don't look so different). Quark was an interesting character, and the Ferengi an interesting race precisely because they are so different from us. The Ferengi went on to lose all credibility by turning into poor copies of humans, and with it the world they inhabited became polluted.
These examples are from long-running series, but the grander scale serves only to amplify a point which applies to all writing - let your characters develop, but not so much that they cease to be what defined them in the first place. Don't let Wooster get married. Don't let Elizabeth become soppy. Don't let Frankenstein's monster join the Reform Club.
