Apologies as this shades into "game modifications we'd like to see" territory but this all got me thinking so I'll roll with it.
It'd be amusing to see a once in a lifetime fortress announcement, or event rather, that adds something rare & FUN. More so from the guaranteed "What the hell just happened?" forum messages that would be forthcoming. Events that played on some chronicles from forums for posterity's sake would be a gas.
An internal counter that kept track of total constructions? A few hundred to few thousand accumulated & there's a one time chance you'd get an "Earthquake!" or "Everything is melting!" announcement if you went hog wild with ice.
The "Boarmurdered Elephants!" thing could involve a proper siege / camping group who drop a pile of *<<*Cave Spider Silk Socks*>>* followed by about 1/3 of your fortress succumbing to a quasi strange mood "I require socks!" & doing anything & everything to get at that pile. Including digging / de-constructing their way out - orders, burrows, & tantruming overseers be damned.
"Keas!" should be followed, with the game first checking available processing power, by a slightly modded horde of 3,500 who scream in. They proceed to nab everything - stockpiles, garbage dumps, locked doors, penned livestock, bridges, cats, worn clothing, the on-screen FPS & idler counters, a baby or two, & that siege of husks that you've been trying to figure out what to do with for the last three seasons. All gone, followed up with a season or three of quietude to recover.