This would definately be one of those, "As long as I can think up good stories then I'll keep writing about this character," much like Dresden or Atticus of the Iron Druid Chronicles.
I've not read the Iron Druid books and can't speak to their pacing, but Jim's said a fair bit to indicate that the large-scale story structure of the DF has been to some extent defined since before they saw print; there's new cool stuff coming in to them as he thinks of it, iirc Changes was around book ten on the original plan, and there was the DB/PG swap, but I do get the impression Jim knows where he's going in the BAT and the major beats that will hit between now and then, so I don't think of the DF as "will keep writing more about this character as ideas arrive" rather than following a larger plot that will ultimately have a defined resolution.
But sometimes in books, sort of like in The Dresden Files, there are enemies or enemy groups waiting in the shadows for their time to shine, for example the Black Council.
I'd note, looking at the DF, that both SF and FM are fairly standalone; had the series not done well, I don't think only having those two of books, or just SF would have felt like a drastic lack of closure, and it's only in GP that the larger-arc elements get foregrounded. (I'd also argue that there's a notable change after DB in terms of how standalone the books are, and how much they feel like chapters of a larger ongoing arc, but I read that as having got from the beginning to the middle of the overall story.)
Indeed, thinking of the DF as a whole as a detective story, the Black Council were introduced in book 8, or a third of the way through, so I'm inclined to think we could be fairly close to a big reveal that Harry's totally wrong about there being a Black Council behind everything, and that their purpose in the series-scale mystery-plot is "initially plausible wrong hypothesis that the detective chases for a while and gets into deeper trouble thereby".
This Villian I've been cooking up has a lot of history with the hero and would almost be like a, "look in the mirror and see your dark side," kind of character(Still tinkering with that), but I don't want to throw too much oomph into Book 1 and then have this villian be less interesting to readers down the road when he returns.
How much of that history would you want to get into book 1, then, and how much could legitimately be saved for later volumes ?