DEADLY FOG
The FOG :
The fog would be deadly. Anyone within the fog would die.
The fog would cover X% of the map (>=25% to be effective).
The fog would move once every X days.
Pros :
The map would be reduced, but no part of the world would be destroyed.
People would need to move regularly.
Localized ressources would be unobtainable for a short period of time, making trade happens.
Cons :
Local banking
Holdings
PS : This is not a troll

, map is too big!