Welcome Bilgecankose!!
These are awesome ideas! but I don’t think they will ever get on multiplayer, sadly (or not). AoE is all about consistency, the game providing you a way to understand what is going on in between hundreds of simmultaneous units in action with only a glance. These implementations could get on the way of this design philosophy.
On this example, a unit with slight differences on clothing/armor and weapons could cause some confusion about the unit ranking. Is that a standard swordsman or a elite one?
Even though, I agree with the general idea of more diversity for some units, I even made a topic to ask for more ethnic correspondence.
On the other side, I can’t see why this couldn’t be implemented on campaings and custom scenarios some more. For those situations there is no hurt, as others have already spoken.