Does it bug you when a UF series is open-ended and has overarching plots but no defining goal?
For example, in most series in other genres you'll have a hero with a Big Goal. Every book, he or she gets a teensy bit closer to completing their Big Goal by accomplishing necessary Small Goals. Once the Big Goal is done with the series is over.
An example would be Harry Potter: kill Voldie. Once Voldemort died the series was over.
If a series is open ended, each book is just another event in the hero's (seriously crappy) life. Something completely new drops into the hero's lap every now and again and by dealing with it, the overarching plot(s) hanging in the air get(s) developed a little. As there's no set goal, the current problem can be any issue the series' basic premise allows. I think the Dresden Files falls into this. The story can go on indefinately as long as each book answers hanging questions and raises some very good new ones.
I'd like to do the latter andI think that's how a lot of UF does it but I can't be sure because I'm only up-to-date on the Dresden Files (I've read about 2-4 books into almost twenty series. I'm currently re-attempting Magic Bites by Illona Andrews). I'm avoiding doing the former type of series because if you drag a clearly defined Big Goal out for too long some readers will get upset, but I a) don't want to be constrained by a singular series goal and b) I need the freedom of a number of books to pull off the metaplot I'm planning.
What do you think?