Just take a look in this footage,and u will see the two points.
First is the way of the Romans engage in battle,probably this HBO Rome,non accurate in many points,the series represents the realistic battles,its plausible to think that the firsts lines of battle retire and are replaced by the subsequent lines.
This is much more realistic than let the soldiers of the front die to be replace by others,its a army in war not a bunch of condemned people in the front. CA could represent it in the game,giving this kind of animation?
The other point is the barbarian armies. Most of north Europeans barbarian dont had hermetic units using all they the same equipment and weapons,CA must let us the players to made custom units,spears ,axes and swords are mixed in any warband,this could broke the unit scope numbers ,but its the reality.
Re: Animations,Units,game play vs realism(immersion).
Technically, that is called "mutatio" in latin, in english could be trasnalted as "turn-over". It's never been reported by any historians at the time, but it's plausible that it's been in use by the Roman army, considering that no one can fight for hours long in the first line . A good way to keep own milites fresh