A few suggestions that will make the game a tad more difficult.
All the following are, unless otherwise noted, very rare occurances.
1. Dormant Volcano Eruption
Fire and smoke rain from the sky as a nearby volcano erupts. Chance of a magma vent appearing in the area. If magma vent/volcano tip already exists, magma vent refills and overflows, as well. Unlikely to occur in areas far from volcanoes.
2. Earthquake
The earth is ripped asunder. A chasm is formed in the area. If it passes through existing dwarf tunnels, it separates them. It could also have an opposite effect, where land is pushed together. This is far more deadly as those caught in the squeezing earth are crushed to death.
3. Tsunami
A giant wave comes over and crashes over the fortress. Water drains down into the water source and, if possible, into the fortress. Not possible on non-coastal fortresses.
4. River flood
Too much rain has caused the river to overflow with water. No river = no chance.
5. Plant disease
A disease spreads through the zone, killing touching plants and trees. No plants means no chances. Does not affect seeds.
6. Ancient Tomb
Your dwarves dig upon the tomb of an ancient king/queen/hero/avatar of a god. They find money and great tombs to put nobles. May be guarded by undead and/or demons.
7. Vein Discovery
New processing technology has revealed that a common stone actually contains a rare metal. A bit higher chance than normal. Will only occur if you dig.
8. Ancient Temple
Your fortress has been founded near an ancient temple. The temple holds a masterwork statue made of a precious metal, and masterwork chairs. However, moving the objects increases the likelyhood of...
9. Wrath of a God
Your dwarves actions have angered a god. Fire rains from the sky, tsunami, chasms sprout out, demons, and so forth. If you survive, however, good tidings may come. Angrier dwarves, high body count, high dead animal count, high dead friendly count all contribute to increasing the chances of this happening.
10. Present from a God
Your actions have appeased a god. You might get food, weapons, crafts, rare metals, metals your nobles like, rock crystals, and so on. Happier dwarves, high money count, and lots of food make this more likely.
11. Animal Stampede
A group of animals are passing by your fortress. In their wake, grass is turned to mud or dust. The animals could be anything, and it provides a great time to hunt. They'd leave after a time.
12. Population boom
The extra food, space, and free time has left your dwarves more time to enjoy life. And each other. Children are born at a higher rate for a while until the causes of the event run low.
13. Erosion
Constant water is eroding some of the dirt and rocks away. Only occurs by rivers and oceans.
14. Mass Murderer
One of your dwarves has gone insane. He or she isolates other dwarves and kills them, before fleeing. VERY low chance.
I know some are unusable, but give it some thought?
Edit: Here're some more.
15. Ancient underground cave
Unlike the cave that already exists ingame, this one has underground plants and creatures (Namely, descendants of dinosaurs). It is completely isolated from the outside.
16. Landslide
An outcrop of rock has gotten too top-heavy and is collapsing. Surprisingly minimal damage to anything inside and not connected to the collapsing thing, though exits will be clogged.
17. Molemen
'Nuff said. Might be an FPS killer, though, since it's basically a second fortress. Ally or enemy, though?
18. Discovery of a maddening item
An item, say an offworldly beautiful statue or a strange flower, causes insanity on dwarves whom stray too close.
19. Pirate's treasure
(Islands and coastals only) Chance on upper layers to dig up a bin containing a lot of coins and cut gems, as well as some crafts.
20. Survivors
Humans/elves/kobolds/etc that have survived a perilous encounter come to your fortress for aid until their respective caravan shows up. It's up to you to decide what to do with them.
21. Dwarves leave the fortress
Some dwarves may leave to form new fortresses, work in different ones, make an inn along trade routes, and so on. About 1 - 20 dwarves leave, depending on how happy they are.
22. Civil war
When a higher-up and a lot of lower-ups have long-term disagreements, the Fortress' dwarves split in two, each side supporting one side of the view. Control one side to try to re-claim your fortress.
23. Festivals
Harvest festivals, victory parties, rememberances of major events, and so on. This would have a set date and would have a cause, so a standard chance of happening.
24. Drought
A severe water shortage goes so far as to dry the lake down an entire level, makes aquifiers temporarily stop giving water, and all rivers, ponds, and brooks all lose one water depth level until it ends.
[ November 25, 2007: Message edited by: Flok Speargrabber ]
[ November 25, 2007: Message edited by: Flok Speargrabber ]