Currently most nations have only a few non-peacefull towns, and vast majority of towns are peacefull, to the point that some of the strongest nations have no more than 2 non-peacefull towns.

The solution to that problem would be to make peacefull towns unable to join a nation if they are not in occupation range of a town belonging to said nation (Non-Peacefull towns can join regardless.) That way, Expanding the nation would require you to set up or claim a non peacefull town before claiming the region around it, which would make each "region" attackable. Right now, nations such as Thracia, to my knowledge, dont have a single non-peacefull town on a continent that they are "contesting", meaning, if anyone was going on campaign drive Thracia out of Africa, they would need to fight them in Europe which makes no sense at all.

Such Mechanic would also make peacefull towns have no choice but to be occupied if their nearest non-peacefull town got captured.