Continental Drift: As years go by, continents slowly drift...What once was a tropical island may eventually become a frozen tundra. Similarly, what once was a glacial cap may become nothing but a big puddle...in turn raising ocean waters across the entire world.
Global warming: Dwarves are not very Eco friendly, eventually that Ozone layer is going to fail with all those furnaces.
Erm... Continental drift takes tens of thousands of years to produce noticable changes. Doesn't really fit into the game's time frame.
On the issue of global warming, you fail ecology forever. Ozone layer has no connection whatsoever to carbon emissions, which are the leading modern-day cause of global warming. The ozone gap is a separate issue altogether.
And as others have pointed out, a pre-industrial civilization has no means of producing greenhouse gases in quantities that would have any significance.
Natural Gases: these could be explosive/toxic and serve multiple purposes. There could be things such as helium, ethanol, ect. Stumbling into a chasm releasing toxic gases into a fort could add some new and fun challenges to overcome. I'm sure there are more things to do with gasses and other interesting examples but that's one idea. Similar in function as Miasma/steam however it can be collected by skilled dwarves?
Melting point: We have flammable materials why not make some materials have a melting point where they become liquid at certain temperatures. *Warning: Dwarves may melt at high temperatures, haha*
There sort of already is a mechanism for natural gases, although the current implementation only occurs in evil biomes. As for combustible gases like coal gas in the mines... would you really
want that?
> Urist McMiner cancels Dig: hit a gas pocket.
> Urist McPeon was caught in a blast of exploding gas! x100
> Your fortress has crumbled to its end.Also, the temperature is already working (well, sort of; fire and heat damage is still kind of broken).
Earthquakes, volcanic eruptions and tsunamis, however, might have potential for implementation. Of course, there's the issue of such events affecting the game world on a larger than current embark scale...