Hello!
How about an undwarvish tower section? If you still have space for it, creating new towers for it would be ideal, but having several existing towers increased in height would also probably work.
Now, the important thing about that tower section would be that there are no internal stairways, or maybe just occasionally one floor connected to one neighboring floor - and that's it.
Instead, there are wooden floors (effectively free-floating hanging bridges) connecting different towers and occasionally including one or more stairs. These high roads could sometimes have crossings or whatever. The important thing would be to have it somewhat free and chaotic so that the floors seem like the strands of a spider web (although preferably not as ordered as those).
Building material for the towers could be anything, stone/stone blocks probably being best.
If you feel really nasty, make sure the floors are used in such a fashion to have dwarves walk around on the open bridges A LOT (but do not blame me if they drain your FPS in vengeance :) ).
Looking through your list, I also see that you do not have a labyrinth. This would be ideal for an underground construction. This being the 3D version of DF, it should be a 3 dimensional labyrinth with lots of dead ends and a difficult path to the target room. (Actually, I think that is a feature other players would enjoy with their adventurers if you chose to share your save with others after abandoning).
Another idea that popped up would a tedious exploit of a bug. IIRC, there had been a bug that constructing a wall and then removing it would destroy a floor engraving. If that is still the case, you could make dedicated halls with special engravings around a specific theme. For instance, you could have a history room where all floor engravings refer to historic events (requiring you to erase all other engravings). Or have a room dedicated to certain creatures (like a unicorn room a.k.a. tomb, or a cat room).
Deathworks