I was thinking a good idea could be a promotion and reward system for successful units. A unit that is able to win 5 battles could possibly be rewarded with some sort of rank (Lieutenant, Captain, Colonel, etc.) and bonus ( +1 strength, +1 move, +1 to one of the bonuses the unit already has or possibly a +1 leadership gained from being a veteran of battle). Every 5 battles could give the unit another rank and another bonus, the unit could also receive some kind of medal or insignia that shows what rank the unit is.
There would have to be rules in order to limit what units can receive promotion. Here are a few of my ideas.
Does not apply to Heroes.
Does not apply to units stacked with a Hero, unless the rank was given before being stacked with a Hero and then could not rank up as long as unit is with Hero.
Wins do not count when capturing an unoccupied building.
Wins do not count if you are defending a city.
Wins do not count if your chances of winning the battle are over a certain %. (A stack of dragons getting wins by killing crows and scouts).
I’m not sure if this is something that is feasible, but it would be cool to have veteran units get rewarded for surviving through a long campaign.