I really hope CA will implement all the tactics the Romans employed in terms of formations and animations. I would love to see the legions use drill techniques like the front-line unit rotation. (See video @0:36)
It could be for example, when there is unit cohesion and drill is performed, the legionaries gain huge offensive and defensive bonuses, and can slice through the enemy. But if cohesion is broken (say the legion is charged on the flank by enemy shock troops, or has been hit head on by elephants or chariots) the unit loses its ability to perform drill in the chaos and loses much of its combat effectiveness.
These sorts of drill techniques should be mainly be automatic as opposed to requiring micromanagement. Maybe testudo can still be left up to the player, but for example if a unit is idle, under arrow attack, and no enemy melee units are near by, it should automatically enter testudo until the player gives a command or enemy units move near it.