I don't particuarly care for the ones that effect only the city your Sovereign is in. It seems to me that they would encourage players to just sit that unit in a city and forget about it when it seems like your goal is for people to actually use the unit for things.
Are you open to things with pluses and minuses like the traits from Fallout?
Heavy Handed - You have a lower chance to hit but when you do you hit harder.
For things strictly in the plus and minus category:
Charming - Your charm monster (and variations of such) are improved either costing less or working on higher level monsters.
Various "slayer" traits - bonus against specific types of enemies
Flammable - Takes increased fire damage (works for other elemental damage types too if those go in)
Arrow Dodger (shitty name) - Ranged attacks have a higher chance to miss against the sovereign
Scoutmaster - you start the game with a basically equipped scout
Thrasher - x% chance in combat for an attack to hit twice
Road Speedster (bad name) - Your sovereign travels faster on roads in your domain
Trader - Your sovereign receives bonus income from trade