When a district is empty for 7 days, in an otherwise non empty city (or under 7 days 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.

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.

A district that is occupied under 24 hours does not count as resetting the timer.

These dates are no longer negotiable.  Plan for them to happen at that time or shortly thereafter.

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.