Let me start out with some disclaimers...
1. I know the game is getting to a late stage
2. I also know there is some upgrading possible with units BUT..
I think it would be really cool if I could create Knightly Orders or Mage Societies etc, and then promote my units and heros that have performed well into these orders and societies. If you look at fantasy literature and RPG literature you find all kinds of brotherhoods, secret societies, etc, that belong to fantasy kingdoms.
I know it's possible to create a unit and call it anything I wan't but, I don't think it's possible to change an existing unit into another existing unit during play. If it is then, this may not be needed.
At any rate, these kinds of things gets the player(me) invested into the game in a way that pre-built stuff just doesnt.