As "real" battles are often decided through tactics, I thought flanking would be ideal to put into the game. A bonus for attacking from the side and/or rear would add tremendously to the tactical part of the game.
Maybe even add units with an ability to "sneak" and giving them bonusses to flanking.
Criticals would work here as well, actually - higher chance for a critical to occur, when attacking from behind and/or sneaking