Since ancient times good military units acted as a whole, developing certain tactics and traditions.
It would be nice to see a mercenary company getting certain traits as it gets renown and brothers get battle experience.
Trait would apply to every brother as long as he spends a certain time fighting with the company, learning from the veterans. Going from 10% to 100%.
Orc Busters – +20 damage and +50% armor damage bonus vs Orcs., but worse morale vs. fellow human race
Counter-guerrilla warfare – better morale vs. goblins, cannot be poisoned, twice as easy to break the nets, but all brothers get -5 HP and get double the bad reputation if fails + something else.
Merchant – doubles the amount of offered goods in the settlements + bigger inventory, yet global map speed is decreased when loaded over certain amount and general damage -10%.
Can be really anything, but should be something important that would define the playstyle.