I would like to be able to build a tunnel back to the mountainhome, by which the visitors from my civ would arrive, that would justify a whole lot of cave adaptation...
Building a tunnel to the mountain home would present its own problems. Where's the mountainhome? How far away is it? How long would said tunnel take to build? Not to mention the creation of a tunnel from wherever you were to the edge of the world, regardless of how many features it'd have to bypass intelligently.
Not to put down your idea, I think it'd be awesome to be able to do such a thing. Thinking of how much it could add to gameplay is very interesting. But with the way this game's development is going, it seems, to me at least, that Toady would eventually like everything to have an actual purpose, everything integrated to be truly part of the world, not just an abstraction.
In this case, if I were developing the game in the same way I see Toady doing so, I'd want a literal mountainhome that this tunnel connects to. Sounds like a huge chore to implement. First the mountainhome itself, then procedural tunnels created from any site in the world.
Back on-topic, perhaps when cycles such as night and day are implemented, cave adaptation detriments could be lessened at night. Of course, they wouldn't disappear completely, because I don't see it as only sunlight, but the wide, open areas and extreme airflows that freak dwarves out.