The most basic change in tactical combat (that is, On the Battlefield) since pre-history has been the inccrease in Span of Control. That is, in Antiquity the commander of the group coud influence directly only those people within the sound of his own voice.
Later, he got subordinates that he could tell what he wanted down and (hopefully) they would take a bunch of people and do it. But still, in most armies in most cultures for most of history the Army Commander, either the monarch or his direct representative, could only directly influence a very small percentage of the troops on the battlefield.
In some cultures and military systems, in fact, he didn't even try. The Greek Hoplite array made no real use of commanders: the strategos simply fought in the ranks with perhaps a fancier plume on his helmet, if that. It was only when 'professionalism' began to creep into the ranks after the Peloponnesian War that we start reading about Army Commanders in Greece actually having some effect on what happened. Alexander at Gaugamela, in his climactic battle against the Persian Empire, commanded the Hetairoi, or Companion Cavalry - 2000 men out of a force of nearly 40,000. He had told everybody what he wanted them to do, as any commander could before the battle, But during the battle, he simply timed the charge of his Strike Force, the heavy lance-armed cavalry, and otherwise hoped that at least the majority of other people performed as desired.
All of which means that an in-game Army Commander should be severely limited in what he can do for most of the game.
This, in fact, would be another place to slide in some real differences among the Ages:
Antiquity: The Army Commander can 'give Command' or move One Unit only while in enemy ZOC. And he picks that unit when the battle starts, and can only change if the unit is destroyed. And if it isn't in the game already, being with a unit that gets destroyed should produce a substantial risk that the Army Commander goes down with the unit.
Exploration Age. The Army Commander starts with the Antiquity limitations, but some Promotions in the Leadership line allow him to Command more than one unit or provide bonuses to either one unit, or several units, or the entire Army.
Modern Age. The Army Commander can dance about the battlefield as you like: with wire and especially radio communications and powered transportation his reach stretches to all parts of the battlefield - until, of course, he talks too long on the radio and gets targeted by enemy artillery, missiles, UAVs, special forces et al.