Competitions and contests
Competition events in which the dwarves could participate, such as wrestling matches, archery competitions, poetry, painting or cooking contests. A dream to win a contest for dwarves with a competive personality trait, with a possible limit of minimum number of participants, or no dwarves being barred from participating? Event could attract visitors?
Farming and growing complexity
Perhaps some additional subterranean crops?
Environment and plant interactions, some plants requiring a different type of soil, temperature?
So in order to grow a certain type of crop, perhaps it would be necessary to find a suitable spot?
Product refinement levels
Refinement depth of the crafting and production elements could be increased?
Additional look at refined product complexity?
Tools affecting work quality or speed?
E.g. Chisel, pick could affect masonry or carpentry?
Tongs or hammer required for blacksmithing?
Refining additional stone and ore types into various products?
Additional furniture and furniture functions
Cupboards for mugs? Weaponracks usable for storing weapons?
Mining complexity
Mining a staircase straight downwards is perhaps too straightforward and easy. A possible way to increase mining complexity could be based on different types of ores, stones and minerals.
The end result could be that it would become possible to try and examine and find seams of ores that are easier to mine, to progress downwards faster.
Particularly hard rocks might require picks made of harder materials, and/or mining rate might be dependent on tool and mining skill quality?
Mining time of staircases could be considerably increased coupled with the previous suggestion, perhaps?
This though seems related to the pathfinding question?
Perhaps staircases could only be built upwards (on top of a supporting structure?) to increase cavern interaction?
Minicaverns
Small pockets of air, subterranean vegetation?
Building durability and materials?
Perhaps buildings made of harder rocks or woods might offer improved shelter for invaders? Maybe wooden walls could be penetrated in sieges?
Perhaps there could be a tradeoff for construction time and such durability?
Stackable stoneblocks?
Perhaps stoneblocks could be stored in stacks by default?
Rainwater collection and crop interaction?
Maybe it would be possible to collect rainwater (In the absence of a pool)?
Perhaps some crops would require (rain)water to grow?
Technology or "objects as recipes" in the procedurally generated world
I think it would be interesting if the objects available would vary a bit between worlds and games. For an example if there was a coreset of basic objects, and then an additional set of optional objects, which could be invented by dwarves and other civilizations. Such as new furniture, clothing, weapons, tools, ideas even. They could spread in the form of books, travelling craftsmen, traders etc. Maybe there could be a simple check to test whether a knowledge is spread well enough to declare it as available everywhere. Inventions could take place in libraries by scholars, in guildhalls by craftsmen, etc.