This sounds like a pretty Fun addition to DF, though i'd much prefer it to be optional. Sudden incursion of goblin overload would be more fun than what is healthy.
Also, some ideas:
Apocalypse of the Adamantine:Oh hey, turns out those adamantine pillars were SUPPORTING THE WHOLE OVERWORLD. Nice job collapsing it into the magma sea and thus killing all sentient life.
(Might not be global, potentially overlaps with hell)
Apocalypse of the Magma:Whoops, turns out you made one too many big holes into the magma sea. Hope you enjoyed unleashing a supervolcano on the planet.
(Utter obliteration of fortress, magma everywhere, extreme dorfiness overload)
Apocalypse of the Moody Dwarves:Seems like whatever forces that possesses your dwarves have had enough of you walling them in. The entire dwarven population is now heading into a massive collective fell mood, which basically means you've successfully committed dwarven genocide, even if you didn't intend to do so. Lots of bone artifacts for the other sapients, though!
Apocalypse of Armok:Armok finds your world boring. Prepare to be smitten.
(I have no idea how, though. Meteors? Giant raging fist of anger? Thermonuclear catsplosions everywhere?)
Apocalypse of the Burning Sun:Pumping all that water into hell might have seemed like a good idea, but you just managed to cool off and solidify the planets core. With the geomagnetic field rapidly fading away, it's recommended that you stay indoors, or at least pack copious amounts of sun block.
(Temperature outside rises to hotter than scorching everywhere, and stuff gets set on fire if outside for too long, underground creatures (including dorfs) may survive if they don't go outside, though the first few layers below the surface might be significantly heated)
Nuclear Apocalypse:Putting all that pitchblende in a quantum storage and then atom-smashing it doesn't seem like a very bright idea now, does it?
(Clarification: According to our lovely wiki, pitchblende is an ore of uranium. Putting too much of it together (AKA atom-smashing) would result in it exceeding critical mass, and thus massive kaboom (dwarven nuclear bomb, get!))