And to think, all I wanted was a little outline around cities/castles that included the villages attached to them.
You could color them to reflect your relation with that faction similar to Total War, but beyond that I don't think many Total War campaign map features would translate well into M&B.
The "diplomatic features" I had in mind were an expansion of the random border dispute event that can occur in M&B. Essentially, this event would mark a village as contested territory on the map and it would break off from the faction that owns it for a while, becoming neutral. It wouldn't provide any tax income during this period. After a while, the village will randomly (?) assign itself to either faction. If the faction that had it before gets it back, nothing changes diplomatically, but if they lose it then the relations between the two factions will take a hit and they're more likely to go to war.
Of course you wouldn't want a "border dispute" to occur at a village that isn't situated geographically between two cities/castles, so you would need some sort of selection process for deciding which villages are valid for border disputes. It could be a simple list system, where each city/castle has a list of "nearby" villages, and these lists would have some overlap for all cities/castles. Detecting if it's a valid dispute would simply require a faction ownership check of both cities to make sure they're not owned by the same faction and not at war, and then finding a village that is in both of their influence lists.
This could probably be modded into Warband, really. I haven't updated my Diplomacy mod in forever, so for all I know it could already exist (minus the village ownership reassignment, as I don't know if that's possible or if they're just tied to the larger settlements permanently).