^^^ your mean.
...is average.
Grammar aside (it's "you're", short for "you are", and in my accent that's not even a homonym with the "belongs to you"=>"your"!), history is surely capable of being littered with dwarven strongholds, connected by tunnels, which may or may not be invaded by others (or just abandoned by the dwarves) so there's plenty of scope for other tunnel-lovers to wander in. Whether goblins or FBs. On the whole, the Circus tends to be (intended to be, at least) a deal-breaker for a fort that taps into it, so I don't know whether Clowns should be coming through the tunnels. But maybe the occasional one (as I've seen above-ground on an Adventure Mode occasion, ironically in a dwarven settlement which was just the stairwell-lined hole in the ground and a couple of otherwise blanked-off central access ramp-rooms below the centre. No sign of any breach into the Big Top) would be considered along the same order of dangerousness as solo FBs are.
Old-style dwarven tunnels (which I've not seen many of, since the caverns came about) also used to bridge (sometimes strangely, uselessly or even impossibly) the various rifts, bottomless pits, etc, which existed before the advent of caverns. I don't know if current tunnels would wander through tunnels (but with walled off sides to protect them), but they'd be open to at least flying cave-life falling onto them and causing a nuisance.
So, imagine a SimCity2000 (I think, I may have got my versions mixed up) version of sending your powerlines/water/highways to the edge of the map and getting the opportunity to link them to the neighbouring region (which you could be playing in parallel) in order to export excess power or import much needed water. Except that I like the idea that it isn't the player's choice, but that after a certain amount of Reputation, the Mountainholm engineers, architects and diggers send a spur of the existing tunnel network towards your embark site, there's a break-through made into your bordering tiles (hopefully engineered, by game or external dwarfs, to avoid the spot you're busy venting water or magma away from) and it's your job to make a
properly constructed roaded-tunnel from there to your Depot (or
a depot, at least). Much as per the old "build us so much road, and we'll promote you" thing.
And it might not even be possible to do, depending on where it happens. Or be difficult and need reorganising of reservoirs and things. That'd be part of the fun.
An interesting idea, anyway.
And the other option mentioned might even allow embark into full-mountain (or undersea?) biomes, with the previously given caveat that you wouldn't see the surface at all (except for a "damp" hint for the undersea, of course, when you dug up to it), where an existing dwarf tunnel already existed, or you were allowed to migrate via the caverns. The tunnels/caverns of entry being the 'initially revealed' open space, instead. (Would probably need some recoding, depending on whether Toady normally has "outside==revealed" equivalence, hard-coded.)