How about introducing natural disasters into the world?
East coast North-America isnt the most disaster ridden zone in the world, but a hurricane
and some landslides would be fun. Make it a small chance of setting off a landslide while
digging in steep hills near a river? Oh, the forest has burned down... And maybe a stampeding
herd of buffaloes? Flood. Lightning. These events must be pretty rare since many players would react negatively
to a landslide killing them and their farm/village. Maybe directly controlled by the devs?

Anyone have other suggestions?
I also think contagious diseases should be implemented.
Tuberculosis, pneumonia, influenza, the common cold, syphilis, poliomyelitis, chicken pox and last but not least: small pox.
That way we could have healers and shamans which would try to heal people!