On the subject of global warming, the change should definately be gradual (not WHAM! SWAMP! like in Civ2!). Pollution though should be less obvious globs of orange goo and more wierd smog clouds over cities and stuff.
On the subject of random events in general, I think there's great scope for adding religion into the game. A random event such as the sudden emergence of a new religion which grips some of your people. It could lead to rebellions, civil wars and so on. In Shogun Total War (OK not the best game ever, but anyway) the religion you chose for yourself and your followers was important in terms of relations with other sides and your own people.
For example, let's say player A is 75% christian and 25% muslim. There's a strong Muslim faction there and so a chance of rebellion if you don't build mosques, or whatever. Other Muslim nations might declare war on you if you treat your muslims badly, or whatever. I think there needs to be more emphasis on building and managing your civ instead of killing the others.