In the last few Dwarf Fortress talks, we've heard about alternate dimensions, and the next set of night creatures. We've also heard a lot about things that used to be in Dwarf Fortress but were taken out before the original release, such as elves animating trees, and animals needing to eat. How much room do you think there will be in the hero arc for adding in the new night creatures? When we get dwarves with ambitions (it seems like there are already goals in place for entities in the game now), I assume we'll have dwarves who want to run their own tavern, or arrange their own furniture, or rob the bank. Does Dwarf Fortress have a means to make abstract structures or cause and effect relationships made by the players identifiable to NPCs? The problem is that you might have a bank vault behind two sets of steel doors controlled by levers and on timers (i.e. the lever does not open the door, but when pulled, it turns on a pump that will flood a tank, and when the tank reaches a certain level, a pressure plate will be triggered, opening the doors, after a certain number of ticks), and the dwarf would need a means to identify the cause-and-effect chain up to and beyond "this lever opens this door".
And this, my friends, is where we get dwarf gangs holding your leverpullers hostage to get out the combination to the bank vaults. We'd also have weapons and armour going missing from the armoury, and probably some kind of security patrol. "Mr Urist, let me introduce you to my atomsmasher. As you can see, we have secured all your prized +Gold Goblets+ and -pig tail fiber socks- underneath the bridge. It would be a shame if something were to happen to them..." Of course, if the same kind of "digging enemies" mechanic were added to dwarf gangs, they would just borrow a no-quality copper pick and dig a tunnel.
For the above to work, dwarves would need a reasonable method for patching up tunnels. They would also need a means for other dwarves to identify and report tunnels for investigation.
I personally think digging is a little too safe. There should really be more dangers (optional, of course) than just breaching the caverns or the HFS. For example, your dwarves might accidentally crack open an ancient tomb from the time before time, and release a lich and its RNG'd defenders. These things should not just be restricted to end-game content - it should be possible to find such an object in the first few layers too. It would also be great to have those HFS-pockets from 40d and 23a added back in, so embarking on the right locations would give you even more ways to ruin your fortress.