It sounds like the hardest hit areas of Faerun were in the Old South region, or what was the Technomancy in the 1st and 2nd IR threads here on ENWorld. We have Lantan, Amn, Tethyr, Calimshan, the Vilhon Reach, Chondath, Unther, Mulhorand, and Thay being the most affected.
I *always* envisioned Mulhorand as being like Zamora (from Conan) with an attitude problem of extremis proportions (some of that Tuigan aggression plus add some Imaskari blood and power, plus a harsh climate of hot and cold, and a nation that is a crossroads of trade.)
The new Mulhorand/Unther, a wasted ruin now occupied by the Dragonborn (with lots of Mulhorandi slaves?) seems appropriate. Let the Dragonborn beware ... the Mulhorandi may be down, but out? Don't bet on it.
Thay? Wizards may be dufuses, but with the Dragonborn south and Netheril north, Thay will be one nation under the wizards (miracle of miracles.) With mile high cliffs and massive fortress cities digging down well below sea level. And dragons ('hey dragonborn, come here, get eaten!')
Unther? Squished. Time for the sahaugin and aboleth to take it over and give trouble to these new Dragonborn.
Chondath? Weren't there beholders ruling somewhere around here? Maybe there are beholders ruling here again. LOL. Better than aboleth ... (beholders eat people, nice and clean. Aboleth dominate them into mindless servitors for life.)
The Vilhon Reach? It's still there. It's just that Here Be Dragons. Forget Volo and his maps. Enter for the peril, the pleasure, and the treasure. Cowards need not apply.
Amn is still around, which means those Knights of the Shield are still around (and trying to take over the Moonshaes.)
I'm guessing Amn took over Tethyr. Too bad. That means Baldur's Gate and Waterdeep (without Khelben to protect it) had better watch out. Amn has no use for Northern Barbarians, except as slaves.
Calimshan? Hopefully, gone back to Djinn rule. Djinn were the only competent rulers in the first place. If the Pasha is history, let the historians write about him.
Candlekeep? Probably untouched. If destroyed, probably all it's information and treasures evacuated. Alaundo might not know of the Spellstorm beforehand, but he *would* know what to do when the castastrophe occurred. (And there are places aplenty on Faerun to store and protect historical knowledge.)
Ah, now ... the Shaar. The western Shaar is a big hole in the ground? Give the illithid, drow, dwarves, aboleth, and others time. They'll fill it up in no time. The dwarves of the Great Rift had BETTER get their act together, if they wish to survive their new neighbor. The halflings of the Chondalwood? If that wood is still there, they had better be *capable* halflings and not Shire types, if they wish to survive enemies north, south, and west.
I'm guessing that Aglarond was obliterated in the chaos that took out Mulhorand and Unther. Where would the survivors go? To that woodland in the middle of Aglarond, of course. Coastal cities are useless when the Simbul is dead, sahaugin and Dragonborn are invading, and hordes of monsters run amok.
Thesk? I'm guessing it bows to either Netheril or Thay (would you want your nation caught between these two powers?) Rashemen? Caught between Thay, Netheril, and the Tuigan (who now have full magic plus their attitude to boot.) Those witches had better have their copies of Dragon #47 (I think that's it) and their berserker warriors ready, or else it's curtains.
Murghom? LOL. Probably declared itself the new Imaskari, after discovering vast magical arsenals and artifacts in the Raurin Desert. Ready to take on Thay, Rashemen, the Dragonborn, the Tuigan, AND Var the Golden all at once.
(grins) Hyperboria, here we come. Heh, this even looks like The Land during the book The Power That Preserves (Lord Foul's Armies and Pillagers running amok everywhere.
It isn't the FR I knew. It's changed as much as FR changed in the 1st IR (which was meant as humor only, of course ... I merely compare the magnitude of the change.) It's a wild, savage FR with tieflings, faery, dragonborn, and hordes of monsters new and strange, aboleth empires, and heaven knows what else, IN ADDITION to all the usual chaos and uproar.