I remember one of the things I liked in the original Warlords was that your capital city produced units that were stronger or produced more quickly than normal. Could you do one of the following:
(1) allow extra money to be spent on production of a unit to have it be produced more quickly
(2) allow extra money to be spent on production of a unit to make it stronger
(3) give a bonus to units produced in a capital
(4) increase city income through improvements