I had no idea grazers could actually pick up syndromes from grasses! That's just fun!
Now, obviously, you can have all manner of horrible things occur in a fortress where a particular grass develops. Say, your animals occasionally develop hoof-rot or contract mad cow disease.
Could vermin eaters contract syndromes from vermin they consume, then?
I haven't done anything outstanding recently and what I've used has been said before.
I will say however that creature classes are caste-level tags. Defining male and female creature classes as part of those same castes lets an interaction differentiate between males and females of a variety of species and apply effects based on their gender, or prevent a caste that has an ability naturally from picking up a dupliate. A werebeast can then have multiple genders, and function as a wild animal that can reproduce (part of wilderness populations), and people transformed into one will maintain their gender.
Interactions can resurrect and transform someone in the same step, which lets you transform the dead into spirits of themselves that you can glean information from verbally (if they will talk to you at all) but wont be able to attack you or escape because they just so happen to be immobile and harmless.
A transformation can lead to a creature with body parts that fall off, and in adventure mode one can take advantage of extra butcher objects to summon items via interaction that someone has to learn, or earn, rather than a reaction that will always appear in the list.
Also, don't forget you can duplicate the material emission interaction itself and make variants that fire multiple projectiles at the same target! These are examples I use:
[OBJECT:INTERACTION]
[INTERACTION:SPELLCRAFTS_MATERIAL_EMISSION_3X]
[I_SOURCE:CREATURE_ACTION]
[I_TARGET:A:MATERIAL]
[IT_MATERIAL:CONTEXT_MATERIAL]
[I_TARGET:B:LOCATION]
[IT_LOCATION:CONTEXT_LOCATION] <-- this is always the emitter for now
[I_TARGET:C:LOCATION]
[IT_LOCATION:CONTEXT_CREATURE_OR_LOCATION]
[IT_MANUAL_INPUT:target]
[I_EFFECT:MATERIAL_EMISSION]
[IE_TARGET:A]
[IE_TARGET:B]
[IE_TARGET:C]
[IE_IMMEDIATE]
[I_EFFECT:MATERIAL_EMISSION]
[IE_TARGET:A]
[IE_TARGET:B]
[IE_TARGET:C]
[IE_IMMEDIATE]
[I_EFFECT:MATERIAL_EMISSION]
[IE_TARGET:A]
[IE_TARGET:B]
[IE_TARGET:C]
[IE_IMMEDIATE]
[INTERACTION:SPELLCRAFTS_MATERIAL_EMISSION_6X]
[I_SOURCE:CREATURE_ACTION]
[I_TARGET:A:MATERIAL]
[IT_MATERIAL:CONTEXT_MATERIAL]
[I_TARGET:B:LOCATION]
[IT_LOCATION:CONTEXT_LOCATION] <-- this is always the emitter for now
[I_TARGET:C:LOCATION]
[IT_LOCATION:CONTEXT_CREATURE_OR_LOCATION]
[IT_MANUAL_INPUT:target]
[I_EFFECT:MATERIAL_EMISSION]
[IE_TARGET:A]
[IE_TARGET:B]
[IE_TARGET:C]
[IE_IMMEDIATE]
[I_EFFECT:MATERIAL_EMISSION]
[IE_TARGET:A]
[IE_TARGET:B]
[IE_TARGET:C]
[IE_IMMEDIATE]
[I_EFFECT:MATERIAL_EMISSION]
[IE_TARGET:A]
[IE_TARGET:B]
[IE_TARGET:C]
[IE_IMMEDIATE]
[I_EFFECT:MATERIAL_EMISSION]
[IE_TARGET:A]
[IE_TARGET:B]
[IE_TARGET:C]
[IE_IMMEDIATE]
[I_EFFECT:MATERIAL_EMISSION]
[IE_TARGET:A]
[IE_TARGET:B]
[IE_TARGET:C]
[IE_IMMEDIATE]
[I_EFFECT:MATERIAL_EMISSION]
[IE_TARGET:A]
[IE_TARGET:B]
[IE_TARGET:C]
[IE_IMMEDIATE]
[INTERACTION:SPELLCRAFTS_MATERIAL_EMISSION_9X]
[I_SOURCE:CREATURE_ACTION]
[I_TARGET:A:MATERIAL]
[IT_MATERIAL:CONTEXT_MATERIAL]
[I_TARGET:B:LOCATION]
[IT_LOCATION:CONTEXT_LOCATION] <-- this is always the emitter for now
[I_TARGET:C:LOCATION]
[IT_LOCATION:CONTEXT_CREATURE_OR_LOCATION]
[IT_MANUAL_INPUT:target]
[I_EFFECT:MATERIAL_EMISSION]
[IE_TARGET:A]
[IE_TARGET:B]
[IE_TARGET:C]
[IE_IMMEDIATE]
[I_EFFECT:MATERIAL_EMISSION]
[IE_TARGET:A]
[IE_TARGET:B]
[IE_TARGET:C]
[IE_IMMEDIATE]
[I_EFFECT:MATERIAL_EMISSION]
[IE_TARGET:A]
[IE_TARGET:B]
[IE_TARGET:C]
[IE_IMMEDIATE]
[I_EFFECT:MATERIAL_EMISSION]
[IE_TARGET:A]
[IE_TARGET:B]
[IE_TARGET:C]
[IE_IMMEDIATE]
[I_EFFECT:MATERIAL_EMISSION]
[IE_TARGET:A]
[IE_TARGET:B]
[IE_TARGET:C]
[IE_IMMEDIATE]
[I_EFFECT:MATERIAL_EMISSION]
[IE_TARGET:A]
[IE_TARGET:B]
[IE_TARGET:C]
[IE_IMMEDIATE]
[I_EFFECT:MATERIAL_EMISSION]
[IE_TARGET:A]
[IE_TARGET:B]
[IE_TARGET:C]
[IE_IMMEDIATE]
[I_EFFECT:MATERIAL_EMISSION]
[IE_TARGET:A]
[IE_TARGET:B]
[IE_TARGET:C]
[IE_IMMEDIATE]
[I_EFFECT:MATERIAL_EMISSION]
[IE_TARGET:A]
[IE_TARGET:B]
[IE_TARGET:C]
[IE_IMMEDIATE]
Replace the reference to material emission in an interaction with a reference to one of your custom ones, and you're flinging more stones at the same time. It can very easily become overpowered, though.
Regional interactions can also do things like resurrect the dead rather than raise zombies.
[INTERACTION:GOOD_RESURRECTION_EFFECT]
[I_SOURCE:REGION]
[IS_REGION:GOOD_ONLY]
[IS_REGION:SAVAGE_ALLOWED]
[IS_REGION:ANY_TERRAIN]
[IS_FREQUENCY:100]
[I_TARGET:A:CORPSE]
[IT_LOCATION:CONTEXT_REGION]
[IT_AFFECTED_CLASS:GENERAL_POISON]
[IT_REQUIRES:FIT_FOR_RESURRECTION]
[IT_FORBIDDEN:NOT_LIVING]
[I_EFFECT:RESURRECT]
[IE_TARGET:A]
[IE_INTERMITTENT:WEEKLY]
[SYNDROME]
[CE_ADD_TAG:NO_AGING:START:0]
[CE_REMOVE_TAG:NOT_LIVING:OPPOSED_TO_LIFE:STERILE:CRAZED:START:0]
[CE_BODY_TRANSFORMATION:START:0:END:2]
[CE:CREATURE:FAIRY:FEMALE]
This particular one will also have the effect of resurrecting the skins of your mules you just butchered, causing them to wander around being friendly, whole mules, which you can kill again (though they wont be pets). Or at least, that's what happened last time I tested it.
You can also have blizzards and dust storms that roll through as creeping dust clouds and cause minor damage to those caught in them.