Hey how about putting a connection between ash mountains and arnoth mountains? It seems to me that there's only one traveling method to the NOrthern Coalition City and its through the woods. Why not make an underground way going there? NC seems isolated enough, at least make it more open hehehe. This could probably be part of the underdark in NS4 because at the moment, i think there's like only 2-3 maps called underdark. So why not expand underdark (since afterall, it's suppose to be huge underground caverns) and make it connected to both ash mountains and arnoth mountains? Also, you can add a story like somewhere in the underdark lives a lich, an evil necromancer, or a skeleton king or some evil boss who creates all those undead skellies, zombies, ghasts, etc that keeps launching attacks at the NC territories
It's just a suggestion. But such an addition would add more underground maps and excitement for players.