I was about a quarter of the way into Structures of Earth when I put it aside in late 2015 to write a different series: the four books of The Chained Adept, the 4th and final book of which was just released.
Why would I do that, interrupt a series?
Well, there were several issues…
I intend for The Affinities of Magic to be a longish series, the way a detective series is. The characters will grow over time and their role in their world will evolve, but the focus will be almost exclusively on each individual story rather than an overall series arc. There are long-term issues that drive some of the characters, and those will show progress (or setbacks) across the books, but not an integral sort of series arc, the sort you're accustomed to in a trilogy, say, where you must “destroy the ring!” or achieve whatever goal you started with.
Some detective series work this way (the ones that don't just reset the characters with each new entry). Each book has a problem to solve that is important to the characters, but in the background the hero is getting older and having problems with his marriage, and an old flame shows up and then goes away again, and he gets wiser (or not) about how to handle certain situations, and so forth.
In The Chained Adept and The Hounds of Annwn series, there's an overall story arc to the entire series, though each book is a complete story of its own. For the former, that arc is probably completed, but for the latter — who knows? Might get longer. I have ideas…
World building to last
Creating a series like that meant I had to approach world building and character building (especially teams) for the long haul. Longer than four books, anyway.
I put some serious work into the technology of magic underpinning the “rules” by which the guilds (and others) operate, since the fundamental premise of the series is that they follow someone who is the “young Tom Edison” of magic — someone who figures out why the rules work the way they do. The whole series background is set in the resulting industrial revolution caused by these discoveries, with our hero at the center of it. I've always enjoyed technology of magic issues — perhaps that's a reflection of my fondness for hard science fiction.
All that is very interesting, but you can't tell stories about concepts. Stories are about people.