What about a more extensive use of historical_friend modifier in some cases like preventing Poland from bashing Hungary every game.
As for the border friction itself,I think it should be dependent on the movement of borders.The longer two countries have their borders unchanged,the less border friction they get.When a change does occur,you get an increase in it,depending on some factors.It could work as AE,so that after borders have changed,it starts slowly decreasing again.Obviously making claims would also slightly increase it.