When it comes to political rivalry and tensions, even armed, in the Realms, I think Tethyr would be a good fit: in its past it has undergone civil war, secessions, internal rivalries; and it has a feudal system more so than some other Realm places -I guess no one remembers this, but I was the first one to guess in these forums when the Waterdeep adventure was to be released. I hope I am right again, because Tethyr and the Lands of Intrigue are among my favorite Realms regions
The Lands of Intrigue would be cool, and they border and trade with the Border Kingdoms so the two books would fit well together.
Actually Calimshans cities and towns also have huge rivalies and even recently were at war with each other until the Chosen of Ilmater liberated them from Genie Rule.