I think an expansion that focuses on disasters would be great. TS4 already has some elements of disaster like the volcano, somewhat, but I think expanding on this would be great, especially for storytelling. Some of the things I would like to see include meteor showers, earthquakes, tornadoes, etc... It would be nice if they would affect the buildings as well. For instance, a meteor shower strikes, and a few buildings get damaged or completely destroyed, and a few sims die. Of course an option to disable certain things like building damage, sim death chance, etc... would be also important for such an expansion.