When an entire city is empty for more than 7 days, NPC Families move into the center district of all other cities.  They will progressively get harder as time goes on.  When a family sets up in the empty city, the difficulty level stops getting harder.

When an entire city is empty for more than 14 days, NPC Families move into every district of of all other cities. They will progressively get harder as time goes on.  When a family sets up in the empty city, the difficulty level stops getting harder.

When a district is empty for 7 days, in an otherwise non empty city - an NPC family is set up in that district. They will progressively get harder as time goes on.  When a family sets up in the empty district, the difficulty level stops getting harder.

These events can be postponed if plans are in motion with hard dates and names as to who is setting up within a reasonable timeline and justification.

These events can also trigger Container Crews if I feel that the game is at a state where there are not enough high ranked people to become CLs on their own.