We are in the Delenda Est section  (for now ).   By this I guess you mean how they would look when promoting? That would be great, I think I mentioned to @Emacz long ago that their armour shouldn't be changed in the promotion system, instead some other extra equipment should change, like crests, cloaks, greaves… while the armour being left connected to their corresponding techs (it's not nice that both things are so disjointed in vanilla). Of course it would be more realistic if techs unlock unit variants instead of just changing stats, but as @LeJardindEpicure says, it would change quite some things. I'm not against that, it's just a different approach, but what I proposed falls in the middle of both worlds: you already have the units, just a bit more differentiated according to armour "weight", and each has corresponding techs that change their armour (stats and looks). If, in some particular cases, some tech allows for the production of something quite different, then a new unit could be unlocked (I'm going to mention something about this regarding siege machines in my post of the Athenians, since I ran hard into this problem some time ago).

In Classical Warfare AEA it provides a bonus: Compact formation of Melee Infantry creating a shield shell. + 4 Pierce and +1 Crush/Hack resistance but movement speed reduced by 60% speed

Please Marius i need this, make the shields out of wood instead of cotton

see also  

If this is for the main game i strongly advise against doing that. It overcomplicates the mechanics for nothing and nullify the possibility of upgrading units when already fighting.