Not wanting to clog the forum with two bad suggestion topics I give you two suggestions in one post:
1) tantruming (maybe berserk) miners/(or anybody carrying a pick so you can't affect this by disabling mining job for the miner JUST as he goes mad) have a chance to dig/channel into a random wall. If behind that wall lies lava/water or an untamed GCS silk farm then Fun is inevitable. Engravers engrave crud pix and curse words / mad chutulhuesque gibberish ruining better engravings. Woodcutters chop wooden furniture with axes. Bookkeepers burn their records and reset stocks precision to minimum. Managers should do something interesting too, maybe delete all orders and add random jobs to random workshops for you to clean up before going on their killing spree.
2) A new type of HFS "phantoms" that can ONLY path through walls. That is, when you open the clown car, they can appear in wall squares adjacent to it and can walk through walls (including up/down z-levels, maybe diagonally) but not through open space, water or magma (maybe warm or wet walls even hurt them). They can attack anybody standing next to a wall from that wall like normal close-combat creatures
##...
#&D..
##.A.
##...
where D is the dwarf about to die, while dwarf A is safe if phantoms have no ranged attack (to be clear there's a wall in the same square as &)
Of course, phantoms that are not in the surface walls, for example like below, are invisible:
###...
#&#...
###.A.
###...
So to be safe from them you can build a chamber suspended by floors alone (which they can't pass, because there's no wall) and try to survive there, or station everybody in the middle of a large room as a temporary countermeasure.
(maybe the "phantoms" need a weak ranged attack to prevent marksdwarves from just picking them off from the center of the room without any danger, or just make them hide deeper if they can't path to hurt anybody)
Something you can't see (until it hits you from the wall behind you) is scary, and HFS is supposed to be scary, not just a tougher megabeast to cave-in knock out and cage-trap for a zoo.