Probably not a new idea, but a few random global events would shake up the fairly predictable nature of the game. I imagine these only firing off 50 some odd turns have passed, so they don't cripple new games. They would have an effect and a duration, then expire. Other fun code for it might be events replacing other events when they are called, or some events making other events impossible to happen while they're in effect.

Events:

Plague of Locusts - 8 seasons. All food producing sources produce 1 less food while this event is in effect.

Bountiful Harvest - 4 seasons. All food producing sources produce 1 more food while this event is in effect.

Terrible Weather - 2 seasons to ?. All movement speed is reduced by 1 while in effect.

Stormy Seas - 2 seasons to ?. All sea-bound movement is reduced by 1 while in effect.

A Terrible Plague - 1 - 8 seasons. 5% of global population is killed immediately. Population growth in all cities is reduced by x, where x is a random number of citizens per turn, while in effect.

A Time of Rebirth - 2 to ? seasons. Population growth in all cities is increased by x, where x is a random number of citizens, while in effect.

A Horde of Termites - 2 to ? seasons. All materials producing sites produce 1 less material while in effect.

And on and on.