I took a quick search for this, and found some similar suggestions, (Such as constructions should collapse if built by unskilled builders and things of that nature.) but what I'm thinking of is a bit different. How about stone that has been mined around degrades over time? Unsmoothed stone could degrade at a slow rate itself, while smoothed stone could degrade at and incredibly slow rate. And, through the use of masons and the like, these exposed walls, ceilings and floors, could be reinforced with more stone to keep them standing. I think it would add a fun level of challenge to the game if it was inevitable that some things collapse, perhaps cutting off parts of the fortress if the hallways are not properly maintained. This may even lead to the abandonment of certain parts of the fort, leaving behind cool adventure mode areas. (So that it actually looks like the fort was abandoned in the first place!)
Now, don't jump to conclusions just yet. It should obviously be easily fixable/maintainable, or it would just be frustrating. Perhaps it could even be turned off if you don't want things like that to happen. But wouldn't it be cool to see parts of a long abandoned fortress falling apart? This could even go as far as being put into adventure mode; no walkways or enclosed spaces would be safe if the fort was abandoned for any more than a small amount of time, as chunks of the crumbling structures would constantly loom overhead and bend underneath, a true worry for even the mightiest of adventurers. Thoughts?