I know you don't want more features, but can't you create a "general" system, where each stack has a general who gains experience with the battles, somewhat like in the Total War games?
That would solve two combat problems:
1) Same-unit combat: the general's tactics aren't taken into account. Look at Napoleon for an example, he didn't have a significant numerical or technological advantage in Italy (1796 campaign), yet he managed to defeat Austria, the Pope and Venice in row.
2) Small vs. Big: when, at the start of the game, you're one of the smaller countries, you sometimes can't win due to the enemy's much larger armies.
That would solve two combat problems:
1) Same-unit combat: the general's tactics aren't taken into account. Look at Napoleon for an example, he didn't have a significant numerical or technological advantage in Italy (1796 campaign), yet he managed to defeat Austria, the Pope and Venice in row.
2) Small vs. Big: when, at the start of the game, you're one of the smaller countries, you sometimes can't win due to the enemy's much larger armies.