Just a couple ideas that I cannot find.
1) Shock Waves: Having loud noises, shock waves and vibrations affect the world around them. It would allow creatures to use sound cannon attacks in the confined caves, allow for realistic explosions and the effect of vibrations against rock. It would allow for noise based creatures, such as banshees, and would allow for noise as a trait for animals (howls, roars, growls, hisses) for communication and intimidation. Shock waves can be used to calculate debris falling into fluids, vibrations for new construction destroying beasts, and explosions can allow for hazardous coal dust.
2) Loyalty: Having the loyalty of your dwarfs tracked. Dwarfs with low loyalty will split off from the main group and form their own, secret faction. They will selling secrets, steal, kill and sabotage you for either money or for someone else's loyalty. Loyalty would only be needed for the dwarf above in the chain of command; a soldier would have his loyalty calculated based on the dwarf above him (the Commander), while Commanders don't really have to be loyal to his subordinates. A bodyguard to a merchant or noble would also have a loyalty meter, and would determine how easily you could bribe them in adventure mode. It can also be used in large scale, with everyone being the subordinate of the monarch, therefor, everyone having a loyalty meter.
3) Rebels: Having a large group of dwarfs with low loyalty(moral?, trust in leadership?) will cause them to split into multiple rebel groups, grouping together based on their similar personalities, ideas and training. The rebels will be treated as their own civ, declaring war on civs and so forth. As they grow in power, other civilizations may find the rebel group more legitimate then, say, you. You have to fight them, or let them take over more of your fort. Get enough rebels in one area, or get two rebel groups with radically different background, ideas and personality, then they will start to fight each other. A more manageable alternative to tantrum spirals, as the rebel faction might not be immediately hostile towards you.
4) Cake mix recipes: This came up somewhat in other topics. Have your legendary cooks, brewers and cheese makers make, on occasion, secret concoctions/recipes. These would be unique types of food that only your chef has the recipe to. They would have unique ways to make them; for instance, having a drink made from bonemeal, fishoil and rotten croc eggs would make:
"Urist's Liven Pony Elixer: It is a strong alcoholic beverage, capable of causing blindness. It is an orange-red color, with streaks of pink. Bits of egg yolk collect at the bottom. Bits of egg shells float around. It smells of bog. It tastes of rotten meat and oak wood. It is served warm. It is a cheap drink for commoners.
The chef would be the only one to know the recipe, and will only pass it down to his apprentice. If the chef dies without an apprentice, he takes his secret with him. If the chef defects, then the new civ gets the recipe. If unions are implemented, this would get even more confusing.