I've always wanted to make an elaborate temple complex, complete with areas for the worship of Armok, ritual sacrifices, prisoner disposal areas, a live-in priesthood... the works.
A temple complex is good, but for now, I'm going for a church to the united dwarven pantheon. Each deity gets an appropriate masterwork statue & some chairs for the faithful, etc. The problem is that the RNG keeps giving me crappy gods: Whenever I find a civilization with a cool name, I'll start a few random embarks just to learn the gods that the civ has . . . and there's always at least one real loser, like the god of crafts whose name means "Butterfly," or the one who takes the form of a male
gorlak.
I have always intended to construct an enormous aboveground castle and keep, but never actually get around to doing it.
My current & previous fort, Marblehalls, has an outer curtain wall encompassing about 98% of the 4x4 embark, with an inner curtain for security (and to keep clowns from ever reaching the keep). Now I just need to get those walls higher than 1 z-level. The central keep is 6 stories high and made of solid marble, not counting the towers and (planned) pyramidial glass roof. At least, that's how far I got in 10 years, before I finally got sick of 16 FPS. Reloaded the original embark, trying again with a pop. cap of only 100 this time.