I'm thinking in terms of something that's going to be a series, and mulling on the scale of closure a story has to have to feel complete.
I think the DF is an excellent example of individual books having solid book-scale endings while still having lots of ongoing series-scale plot. But I can't think of anywhere the DF as we have them could have stopped and felt like a complete series.
What I have in mind would be three books, with a solid endpoint there such that if nothing more gets published readers would be happy. And another layer of stuff going on such that if they did do moderately well there would be four more books and then another solid endpoint.
I'm aware that I kind of want two contradictory things, in terms of wanting a book 3 that feels like a satisfying last book and at the same time works as a natural flowing middle if I get to write four more; can anyone think of any examples of things doing that that work ? I can think of plenty that don't; the closest I can imagine to what I have in mind is the jump from book 3 of the Black Company to the Books of the South, and I have not found the Books of the South/Glittering Stone to really work all that well for me or to feel really much of one thing with the first trilogy.