Since the Slave Rebellion is normally in addition to another Event, there are no balance concerns.
If you want to duplicate the "Thematic Rebellion" rules
(B&C rulebook, p. 19), make the Slave Rebellion happen on turns 3, 4, or 5: roll a die on turn 3; on a 1 or 2, it happens then. If it doesn't happen then, roll the die on turn 4; it happens on 1, 2, or 3. And if it hasn't happened yet, it will happen on turn 5.
Tags: Playing Without Events, Kingdom of France