I swear
@gyor , if you guess it's a FRCG setting book every time you really do set yourself up for disappointment...
So I actually took time to listen to the whole podcast here, and I find it pretty interesting that the word Chris used was "towns." If it was a series of Gazetteers spanning the entirety of a setting like FR or Greyhawk, my gut says he would say "cities" or "countries" as the focus would be on the largest urban centers or whole nations.
Chris mentioned the big inspiration for his gazeteers was Wildemount's book, which if you look at it doesn't split up regions by town, but instead by regions and cities. Chris saying town makes me think the scale of whatever this is, will be smaller.
I think this also could apply to an area like Dalelands; he would probably write a gazetteer to a whole country (despite them being smaller), not just it's largest "town."
So I think that Icewind Dale and Cormyr are both very good suggestions, as they have towns within one "country," so the town naming makes sense. Cormyr also makes further sense, as its nobility will each have their own heraldry.
Now, the wildcard here is the OP's original guess, the Outlands. They are of course named "Gate-Towns," though I have no idea if they have heraldry. They definitely rival each other.
My hail-mary guess is this; that there will be a Planescape book at the end of this year, including this Gate-Town gazetteers. There should be a book released this year with each type of Gem Dragon, and their statblock, and a Planescape book seems like a good fit for that material.