Work on the synergy of the Uniques in each of your Civs, for example Parthia has two unique combat units but their UA doesn't give you any incentive to wage war upon others.
I've actually changed their UA a while ago.
I think i'm going to increase the bonus, since all their UA becomes obsolete with mounted units.The Persian Horde:
Capital produces 3 Horses. Each city you own produces one additional horse. Mounted units receive +15% combat strength when next to another unit.