Especially with the introduction of commander visibility into armies and movements, it would be excellent to implement battle outside of towns. Same goes for any kingdom who watches others walk across it. For instance, if I notice an attack, I should be able to intercept it rather than have to send reinforcements to the town and wait for the response. If I can see them marching/sailing across my kingdom, I should be able to confront them no matter what. Same goes for troops passive...If they are marching/crossing my kingdom, I should be able to intercept them and either allow them passage, attack them, ask them to pay a "toll" or deny them entry and force them to cross the long way.
Of course, and then we should implement an army requirement to solve puzzles, because weeding a garden would tire your hero too much if he does it alone...
Dont you think the game has already got enough war aspect? What you are proposing would turn trade, "adventure mode" and even exploration into an annoyance. Why would i want to trade with someone if not just i need to wait hours to get to them, but i also have to worry about not loosing the resources? Why would i want to explore if a minuscule army can destroy 25 explorers without taking damage? And what if you are completelly surrounded by blocking players? Sorry but no. This would turn the game into "lets hope my neighbour is sleeping"
Plus, you can see only your alliance troops when you are a commander, not the other people around you.