one way we used to play, is with what we called 'paratroopers'
with beginning of turn armies, we would be able to place them (all or some) on the board in enemy territory 1-2 territories deep and those battles would be fights-to-the-death and resolved prior to other actions
for instance: if playerA owned iceland, and playerB owned greenland with some absurd amount of armies, playerA could drop armies on NW territories, ontario or quebec - this helped stop the 'wall of armies'
another variant, 'rebels'
when a set of cards is turned in, all territories on the cards receive 2 of your armies, resolution as above