I'd really like to see more detail and variation in not only the soldiers, but their commanders aswell. Perhaps things like that mask could be "unlocked" if that cohort killed X amount of soldiers? I think it suits the purpose of the mask - intimidating your opponent. Or just for units in general, we already know experienced soldiers will look more scarred and hardened than recruits so why not add some more stuff

(experience is gained from kills anyways so there's no need to distinguish the 2)
A few examples
0 Kills - Ordinary armor, gear in better shape/shinier etc.
100 Kills - Shields have dents, units look more hardened and have some scars
500 Kills - The Legion's pattern on the shield becomes harder to see, lots of scratches and perhaps some small holes from arrows(although they would probably be given new shields if their old one had a hole in it) Centurion's armor becomes more decorated and he is given a cape. Soldiers look battle-hardened. Some of them have bandages on their arms/legs.
1000 Kills - Eaglestandard bearer added, perhaps? Some of the soldiers have large scars, a few of them are missing an eye or an ear.
All the way to 5000 kills - Legionnaires are given extremely nice looking "hand-crafted" armor, Centurion gets a helmet with the crest going from back to front, rather than left to right - indicating a higher rank. Also his shield(i assume centutions will use shield) is beatifully made with details and what have you, instead of the regular ones all legionnaires wield. Yes, it's quite alot of stuff but five THOUSAND kills without loosing your cohort is quite alot.
Again this is just an example but maybe this could contribute to their goal of "you should care about your soldiers".