Parmandur
Book-Friend, he/him
True, with bigger books it would work. But still, covering everything from Cormyr to Mulhorand and from Amn to Durpar each in one book does seem to be a lot of ground to cover...
Granted, I could also see them doing just one more "Adventurers Guide" for the (greater) Eastern Heartlands and just leave the rest for a putative FRCS, as the rest of the setting tends to be more "places adventurers go" rather than "places adventurers are from"...
Well, given the treatment alot of those places have received in the past, I would expect more broad strokes and maybe a little less city by city detail, and that would still leave room for deeper description than the 3E FRCS for most of those places. I bet of we took that book, and looked at the page count devoted to the areas in question respectively, that two 256, say, page books would give room to expand, assuming a similar fluff to crunch ratio.
That three-book scheme would also place solid familiar ground core areas in each book: Amn and Tethyr in the South, Coromyr and the Dalelands in the East. Before getting into some of the gonzo stuff.