Another take. After reading much of this thread, and from various other conversations, I was starting to think that the chances of an FR campaign book were less likely than when I wrote the original post. But one important fact has come to my attention that has changed my view: the continued sales success of The Sword Coast Adventurer's Guide, which is currently #16 in all Gaming books on Amazon, or the 9th best D&D hardcovers. Ahead of it are the core books, the splats, and the two recent settings; behind it is everything else, including a bunch of books published more recently. Meaning, it continues to sell fairly well, without the dive that most other books experience after initial sales.
That said, and where the new take comes in, what this makes me wonder is whether they'll eschew a full Faerun setting guide and instead continue with the regional "adventurer's guide" format. Sword Coast came out in November of 2015, after WotC had published four adventure books centered in the North. This makes me think that if and when WotC moves beyond the North for their story arcs, we might see another adventurer's guide book for whatever region they choose.
So I would speculate that we'll get a better sense of what their FR setting book plans are, depending upon the next two or three FR story arcs.
1. They're all set in the North. No setting book.
2. They're all set in another region of the Realms (e.g. Dalelands-Moonsea), then we'll see an adventurer's guide for that region.
3. They're set in a variety of relatively disparate regions that aren't the North, then we might see a full Faerun guide.
Assuming the next story arc is truly set in Icewind Dale, that puts my "2-3" back another story arc season. I think the key is two adventure books in a row--at least--implies a possible region guide. Two disparate areas implies a full campaign guide.
Now if there are two story arcs this fall, one in Icewind Dale and one elsewhere in the Realms, then the next story arc set in the Realms might tell us which of the three directions they're going in.
Maybe.