I agree, and those are all good ideas. I think Toady's kind of stuck between a rock and a hard place, because there's also the camp that says, "I want the complete game experience in every map," and that's why we have adamantine in, well, every map now.
It's true that it requires nearly no skill to dig deeper. 2d DF had a nice little progression of river-chasm-magma-
that worked well from a gameplay point of view, and it could be taken as inspiration. Aquiferous caverns, perhaps even pressurized, would be a a nice challenge-- it's possible to punch through aquifers, although it takes some thinking, and it always gives you that, "fuck yeah, I rule" feeling at the end, which is one of the things that fun gaming is all about.
The idea of an airy cavern with thin, web-like connections between floor and roof is appealing, but it's just too easy to wall off. It would be so much nicer if it could prove a source of constant danger to ANY dwarf that had to go deeper than it.
I think that a change to magma workshops could restore some of the danger to magma workshops. Magma beasties are already very dangerous (fireballs, wtf), but it's too easy to wall them off. Remove the workshop's impassible tiles and suddenly you have a recipe for fun.
I kind of think that adamantine is still too easy, despite the hell risk. I never feel like I'm the only fort in the world that has the wondrous metal. I think that placing adamantine UNDER the magma sea would work for this. First of all, a new player probably wouldn't really wonder what was under the magma sea, they'd just be excited to use magma forges. After a while, they'd get curious, and start trying to figure out how to solidify the magma. Breaking through a magma sea is a difficult task (well, impossible, with the way magma flow tiles work now, but that could change) and I think the player would have a really fun time exploring under the sea, finally discovering adamantine-- wow. That was a challenge to reach. When, of course, the real challenge lies just ahead....
It'd be pretty important for this stuff to play nicely with procedural generation. When every map has aquifer, open air cavern, magma sea, it gets a little gamey. Some adamantine poking through the magma would be fine, as would some maps without aquifer caverns. The point is that every map should have at least one major engineering challenge between surface and adamantine, preferably, before the magma.