It should be nice to give an
Intercept order to an army. If any adjacent province is under attack, the army will defend it (being informed of the attack by the people of the attacked province or supposed scouts). If there are more than one of the adjacent provinces under attack, the one with the biggest enemy army will be defended.
Also, I'd like a second patrol order, let's say
Patrol for enemies , only searching for sneak units in the province, not killing people (and not reducing unrest of course).
It seems fair and not unbalancing imho
.