Slave revolt ideas:
1. An auto-acquire, must maintain effect promo that gives slaves a random chance to acquire a 2nd promotion (rebel?) when they aren't in a stack with a non-slave unit. 2nd promotion converts them to a warrior with auto-pillage and gives them to Bhall.
2. Slave revolt event that triggers fairly frequently, with escalating horrible results based on whether or not you have a garrison and happiness/health of the city.
Level 1: One turn of unrest
Level 2: More unrest, one or more improvements in city radius pillaged
Level 3: More unrest, unhappiness, bad health, population reduced, more pillaging
Level 4: Would only trigger when city ungarrisoned or poorly garrisoned. Like #3 but more so, a number of Barbarian warrior units created in city radius