Medieval II had a system where you could actually see which units had upgraded weapons and armor. The weapons looked a little bigger and the armor looked stronger. For some reason, that wasn't included in Shogun II, Rome II, or Atilla. Would you like to see its return in Warhammer? I'm thinking that the different factions might have some unique visual upgrades in addition to the mundane. For example, bronze and silver weapons might just look sharper and longer.
Here's a short list of two factions that might have some neat visuals.
Empire
Armor Level 1: Half-plate mail for knights and greatswordsmen, padded gambeson for free companies, spearmen, and missile troops
Armor Level 2: Full plate for knights and greatswordsmen, leather for free companies, spearmen, and missile troops
Armor Level 3: Gothic plate for knights and greatswordsmen, brigandine for free companies, spearmen, and missile troops
Weapon Level 3: Blades are etched with glowing Dwarfish runes.
Bretonnia
Armor Level 1: Half-plate mail for knights, leather for men-at-arms, padded gambeson for missile troops
Armor Level 2: Full plate for knights, chain mail for men-at-arms, leather for missile troops
Armor Level 3: Gothic plate for knights, brigandine for men-at-arms, chain for missile troops
Weapon Level 3: Blades shimmer with a faint green or white light, signifying they have been blessed by the Lady.