All that's needed for Planescape is a follow-up Manual of the Planes type book, and at that point, coverage is pretty complete. It seems that the vast majority of people knew that there simply wouldn't be a way to do complete, full, in-depth coverage without a book that would need a wheelbarrow...
The various mentions of the various Mulan deities wasn't in any organized manner; it was much more off-handedly. Also Isis and Osiris weren't mentioned either, and they should have been. It was all very ad hoc.
We kind of need a sister thread to this one: "What areas should Adventures in Faerun II: Elminster Boogaloo cover in detail?"
I've often stated that Cormyr should have gotten in-depth coverage in this book, as well as Thay. Let's hope it does well enough that we can get a sequel covering...
The only mentions of anything outside of Faerun proper by name that I've found so far is a mention of the Hordelands. Granted, there's a lot of reading to be done.
As part of a suggested outline for a Dalelands campaign featuring Moander:
"Levels 17–20. Moander manifests as a Blob of Annihilation to enter Myth Drannor and feed on the ancient magic of the broken city, becoming even more powerful."
I mean, that was basically his plan in the novel, but I...
As per page 238 of the PHB, all spell targets must have a clear path from the caster and can't have Total Cover. Which would make the first problem go away, although a magical scout is still a possibility.
It's been about a decade in-universe since we've had any updates, which seems to be that Unther conquered the the rest of Tymanther other than one city, and many dragonborn went to Calimshan as mercenaries.
I was thinking the same thing. You could easily use the same format to focus on 5 - 6 other areas for several books. I'd love to see this for Cormyr, or Thay, or Mulhorand