I've had bits and pieces of this idea scattered around in other threads, but it wasn't until recently until I thought of an easy way to bring them all together... meaning this might get a bit on the long side. Anyway.
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First off, what problems would this be trying to solve. For one, digging is sort of an important part of DF, but it's very easy, and results in a lot of excess stone for no reason. The excess stone is, well, if you've played the game for any length, a bit of a headache all around, let alone messing with your industrial economy as far as, having way way way more supply than demand. Just hollowing out your fortress is both very easy, and generally gives you more stone than you'll ever need, and usually more than a bit of metal as well.
The second issue being tunneling units, and how overpowered they'd end up being. Regardless of how hard you want the game to be or how fast you think goblins should realistically dig, tunneling units in DF would be hated for about the same reason that creepers are hated in minecraft; besides being fairly powerful, they'd just ruin all your hard work, and that's no fun, especially considering how constructed walls are inferior to natural ones.
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Next, what. Constructed tunnels would be an option instead of digging... like how you can either dig out a stairway or construct a stairway, except for, well, tunnels instead of stairways. The advantage of digging would be that it produces stone or whatnot, it clears the space for workshops, and can be smoothed and engraved, so while you might construct hallways early on, you might dig them out later to decorate them. The disadvantage of digging is that it should take much, much longer... but since you wouldn't have to dig every little thing, that wouldn't be a problem. Digging would be for your established areas, not for exploring.
The advantage of constructing would be, of course, speed. You'd only [/i]need[/i] to dig out areas for your workshops, beds, stockpiles and such, and of course for mining, so constructing would be a quicker way to get to where you want to build your fortress, the initial hallways between areas, and exploratory shafts to find caverns, magma, water, ore, ect. The disadvantages, besides the lack of stone and ability to construct over the area, could be a few things. Constructed tunnels could only show the material they pass through, rather than the area all around. It could also be set up to only allow one-width passages, which could help make minecarts (which as I understand it are a goal) a bit more relevant. If digging is slow, and haulers couldn't pass each other through the easy-to-construct tunnels, then taking the time to set up a way to carry a bunch of stuff at once would be useful.
An exception for the no-building-in-constructed-tunnels would be for fluid drainage and infrastructure, like doors, grates, floodgates, and such. Kind of like how you have wiring and piping behind your walls, a fortress could have water, driveshafts, and whatnot in the easy to construct tunnels.
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As far as enemy tunnelers go, constructed tunnels would let you easily repair after an attack... easily enough that you could have tunneling vermin, and cleaning up the more poorly-tended areas could be as common a chore as hauling or smoothing. Even if something tunnels through three rooms and a masterwork engraving, as soon as the problem is dealt with, someone with a "deconstruct enemy tunnels" task could fix it automatically faster than you could designate the area manually. You could even have animals that make their own dens, similar to how we're getting automatically generated cities, but simple enough that the computer could do it while you're still in the fortress.
On the defensive side, there could be an option after smoothing, along with engrave and fortify, for reinforce, which would make it either harder or impossible to tunnel through, at the cost of, well, not being an engraving. A reinforced floor might also reinforce the ceiling, since there's no way to select the ceiling alone at the moment. For deconstruction, there'd have to be some logic about starting at one end of the tunnel and working your way back, or else dwarves would just seal themselves in. You know they would.
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And that's about it, I think. The simplified pitch for people who don't like to read: Constructed tunnels would be easy to implement, they'd make the actual fortress-building part of Dwarf Fortress more nuanced, and it could be more easily expanded to enable future things Toady wants to do.