My current fortress, with its Dwarven Pit god cult:
Right now I just dump people into the pit as sacrifices every season, to satisfy the pit god so someday it does not crawl from its home to devour me. This is not epic enough. No, the pit gods demand more.
So, I must build a spire from the very bottom of the pit reaching high into the sky, with a tunnel down the middle that sacrifices can be thrown. It will, of course, need a proper alter and an executioner.
I havent decided on the bridge drop method, the "pit" method (which precludes immigrant sacrifices), or the toilet method, where I can see them struggle against the whirlpool before they are pulled into the depths, with a stairway on the opposite side so they have hope for escape. Or whether I would let them escape...
Anyways, after THAT's finished and I start a new fort (the old one destroyed by the pit god some stupid dwarf angered of course) my REAL megaproject will begin:
Hollow out an entire map. EVERY single z-level. Minimize building to neccesseties until then, since it will all be destroyed shortly.
Once complete, move every dwarf underground and then collapse the stairway. NOW the project can begin.
Now I build the undercity, with temples and buildings and towers, as if it was a surface city, but underground. It should be sprawling, think of the city from eternal darkness --- Including the 9 "power towers" and a single central spire.
This spire will be the last part built, and will come up from the ground high enough to penetrate the surface world once more. Then trade and such can begin normally again, my underground civilization having finally "made contact" with the surface dwellers.
Hopefully, I'll be able to do this on a map with a magma pool and a river, for more fun and founding the city in a flooded water/magma pit.