I've noticed that I will have to perform exploratory mining through the earth to find metal veins. This task, while realistic, is tedious and I do not particularly enjoy digging out possibly useful area, making the underground ugly, ecetera. It is even possible to run out of metal veins on a map, particularly on smaller fortresses.
I am also looking forward to the feature of having underground creatures attack the fortress from the depths...
I was thinking, why not tie in the exploratory mining with this feature? If there is a metal ore going to the edge, why not allow players to mine into it and create an 'offmap' mining site; this will allow a constant stream of ore, at a highly lowered rate (say, 1000x the time it would take for the miner to dig out a normal tile of that ore). There could be a chance that there is an invasion through this tunnel and some miners would die, some would run out being chased by baddies. This would make the feature dangerous as well as time consuming to those who attempt it.
The miners would still be effected by hunger, thirst, and tiredness, but the player could not interact with them due to their distance from the fortress. This would also allow fortresses to have an interest in having more than one or two legendary miners, especially those centered around metalworking.
What do you fine gentlemen think?
[EDIT: 10:19 PM] To relate this to another feature, more off-fortress activity would be nice. Why only send armies to invade places from above ground? You may want to send a skirmishing party into the mines, to protect the miners.