Rival leaders
Every rival is a person with a personality, a memory and a plan.
Each rival leader has a style, like a dictator, a democrat or a pragmatist, and traits such as aggression and charisma. Some are spoiling for a fight, others would rather talk.
Rivals remember what you did to them. Some forgive quickly, others hold grudges for a very long time.
They build, expand, trade and make alliances among themselves, just as you do. Some flee when their home is in danger.