Ideas:
- Workplace accidents, aka smelters that catch fire (1% chance for burning product).
- Workplace dangers, aka bacteria that cause sickness in the butchery/tanner, alcohol vapors in the still, mercury/arsenic vapors in the smelter (% for syndromes)
- Vermin bites that spread diseases
- Vermin that eat more or your food
- Raw foods cause sicknesses
- More demands/mandates
- More nobles that require rooms with specific furniture
- evil rains/clouds/mists with delayed syndromes, (unit goes in, nothing happens, unit turns into monster later)
- better vampires (no growing eye teeth, dwarven blood removes NO_DRINK so that they still appear alive and drink booze, more combat power)
- better werebeasts (longer transformation times)
- grasses that add syndromes to grazers, like evil grasses turn grazers opposed_to_life, other rare grasses make them sick
- Use GROUNDED_ANIMAL_ANGER for proper fights that break out between animals. (too many in one area, scratches/bites lead to infections, rabies, etc)
- More flying cavern monsters that path into the fort, for example with BUILDING_DESTROYER or CURIOUS_BEAST
- More world-gen secrets that your migrants might have found, which make them a danger to your fort, like secret necromancers to suddenly turn
- Cavern layer 2 & 3 trees with vaporizing wood that add syndromes. (surface and cavern layer 1 wood is used by caravans, which would bring evaporizing barrels/wagons) For example poisonous mushroom clouds.
- Rare cavern layer 2 & 3 plants that have very strong negative effects
- A lot more mischivious monsters that might manage to sneak into your fort, especially flying ones. Lever-pulling and cage-opening are among the few things that can really hurt a locked-down fort.
- Cavern monsters that affect a dwarf with a syndrome that kills him/transform him into a monster. For example I made face-huggers that can impregnate dwarves, which turn into a green devourer after a while.
- Changing personalities/attributes to make dwarves tantrum sooner.
- Make food harder to get, aka less omnipotent farm plots, forcing people to hunt or trade (aka opening their fort)
- Make drinks harder to get, resulting in more water being used for drinks, lowering morale.
- Mining accidents, rare gems/veins that vaporize and inflict negative syndromes.
- Mini-aquifers, rare gem that spawns unlimited water for even more mining accidents.
- Random god of death - Every year there is a 1% chance that a dwarf dies (so if you have 100 dwarves, on average one dies per year). You could add humorous explanations to the syndrome/interaction that kills him like "fell down the stairs", "chocked on his drink", etc. This is clearly unfair, but could lead to interesting events, depending on who dies.
- Antisocial caravans, rare castes of elves/humans that turn the caravans berserk once inside your fort, simulating bandits that killed the merchants, donned their gear and try to rob your fort.
I can think of many more, but all of the above are possible with raw-modding only.