Create events and gameplay involving prisoners and such.
Decision (aka event) about "what to do with prisoners now?"
Mega-Prison complex on surface; prison labour and prisoner factories and such? Cause auto-enslave (little bit like caste system)?
Go Australia on them and send them to the colonies (either just have them simply as pops, or maybe create a "prisoners" faction to mark them or something like that?)
Create events and gameplay involving prisoners and such.