Ocean-based volcanoes occassionally forming islands.
Bushfires burning through forests in equitorial regions...

Any (simple) others?

I have a proposal for a terrain system, that was originally posted for The List (v2.0)

TERRAIN

First step is determine the form of the terrain. Form never changes during the course of the game.
These are:
Deep ocean
Ocean shelf
Glacier
Mountain.

Flat
Hills
Major Rivers

Navigable rivers cannot be in hills, so to keep things simple they will be their own landform. Otherwise it is basically the same as flat.
Flat terrain gives a food bonus, hills have a defensive bonus and allow mines.

Vegetation:
Vegetation can only be applied to the last 3 landforms. The first 4 do not support vegetation types.

Plains
Grassland
Taiga (Evergreen Forest)
Boreal Forest
Rain Forest
Tundra
Desert
Swamp

Combinations to note:
hill + grasland = plateau
river + grassland = flood plains, the most fertile standard terrain type.
flat + grassland = no irrigation reguired.
taiga + hills = shield, like the canadian shield or siberia. This is the best resource producing terrain.
swamp + hills, acts as fresh water lakes, these cannot be changed to anything else. (just as a place keeping way of programming it)

Landform cannot be changed by your engineers.
Vegetation can.

Allowable changes:
Grassland <--> Boreal Forest
Taiga <--> plains
Swamp --> Grassland
Jungle --> Plains
Irrigated desert acts like Plains.
Irrigated plains acts like Grassland.
Tundra can't be changed to anything else.

Terraforming model.
Economic TI's are auto built by enginners in cities. A former type unit is fortified in a city. While it is there, it can automatically build farms and mines as well as changes the vegetation in friendly squares. You can change the terrain view mode "T" and click on squares to set their priority and change preffered. If villages are used, can only do squares surrounding friendly villages and cities. Squares do not have to be near the city the engineer is in, just connected by roads, or in the same region.

Engineers outside of cities act like normal civX, but they cannot build farms/mines. (to prevent building away from your pop centres). When acting this way they can build paths (along with most millitary units) as well as Roads, highways and RR, and military TI's

Nothing can be built on deep ocean/glaciers.

military TI's:

Bases act as both fortress and airbase, coastal bases can have ships enter as well. Units can be 'deployed' to bases.
Forts act as a scout unit and stop the first enemy unit to step on it. also offer some defense bonus.
Sonar acts as a fort in the water.

economic TI's:
Farms / modern farms, built on any plains, grassland or desert.
irrigation, built on desert or plains, makes it act as the next stage better, without changing it.
mines / modern mines, built on any hills or mountains. Farms can also be built if it is a grassland/plains square.
fisheries can only be built in continental shelf, which usually only extends one square from land, but sometimes more.

transport TI's:
Paths, built by and most military units as well, MP/2 move
Roads, MP/4 move, and give a trade bonus (and allow resource sharing) when connecting cities.
Highways, modern roads, MP/8 move, more resource sharing.
Rail-roads, MP/8, take 50% longer than highways to build, but enemy units act as if on path (or road?) when using.
Tunnels, connect two squares of land under a shallow ocean square. Allows land units to cross water.
Canal, connect two ocean squares over flat land. Can connect a city on a river to the sea if 1 square away.
When a unit enters a tunnel/canal it uses 1 full MP and goes directly to the other side, units cannot rest in a tunnel/canal. Both can only be used by the owner. Neither can be pillaged by sea units.
A tunnel is built by having an engineer on each end of the desired location. Both then build the tunnel. To use you press a key.

By the way, I think it should be entirely possible to have a ocean volcano come up during the course of a game. Only a square of land could be created in the middle of nowhere, so even though it could be slightly unrealistic (ie a square's too large), but it could be a nice touch to the game that wouldn't affect gameplay.

On a related note: Why aren't shipping ports represented in Civ? Sorta like an Air base? Any ideas on this?
An example of a Shipping port would be Guam, Singapore when occupied by the English, or other Pacific Islands (especially during WWII)