Well, a few days ago (April 2 to be precise), I suddenly decided I wanted to play DF again and give my megaprojects another go. And lo do I find a new version! So I'm playing around with it a bit (did a little modding), and getting my regular fortress (got a spot with aquafer+stream on one side, solid rock on the other, and freezing in winter) structure up and running (must make my obsidian factory and start the magma works going). Once that's settled in, I will be creating two super-structures.
The first will be a giant skull overlooking my fortress, jutting out from the mountainside. I haven't decided yet what it will be made of.; been considering obsidian or ice (if it still doesn't melt once made into blocks) mostly. The inside will be hollowed out, the eye sockets made of Crystal Glass, or gemstone windows. The mouth will have a drawbridge (will require some testing). The drawbridge will be closed while I fill the skull, or if I need to perform any maintenance. The skull will, of course, be filled with magma. Normal conditions will have the mouth open, pouring magma out over the (covered) entrance to my fortress, and down into my moat. I'm considering making the cover over the entrance a drawbridge, so that if need, I can pull it back to drop magma on invaders.
My second megaproject will be of a similar idea and construction (hollowed out fiery death):
The atomic boozebomb. A 40 wide cylinder supported by a single retractable column (or some other method for creating a cave-in), tapered at the ends and sporting a tailcone on top, all constructed, again either obsidian or ice, or maybe glass. The body will have floors of booze stockpiles, while the tailcone contains, you guessed it, magma. I'll probably put up a walled path/road/funnel outside my fort so that in order to approach, you have to pass by/under this thing. And someday, I will pull that lever and see how much of the mountainside I can devastate (and if I can melt my stream in winter).