Maybe a separate building ? So that you have to specialize your city. If a building do too much things all of the cities will look the same, becasue you just need 1 building to do lots of things.
You cwant a unit-generating city : barracks (creation), forge(reduce cost for equipment), training yard (reduce training time from perks), Magical forge (reduce training time from equipment), Commander corner (units starts at level 2). You woudln't be able to put all of those in all cities if the maintenance costs are right.