Depends on the fort. Mostly I set up an identical underground stockade, some farms, stockpiles, basic workshops, etc. My main goal is to work above-ground, quarrying and building up stone. These buildings on the surface are usually unique to each fort, expanding as time goes on, repurposing, rearranging hallways and magma-ducts, etc. This adds a lot of flavor to the fort, as I reposition the mist generator and end up with the old waterway now empty, converting it into a hallway and some of the runoff chambers into bed chambers. It can get exceedingly interesting when you let it sprawl out like that over time. Don't worry about the final result, just worry about what your dwarves need now and a convenient way to set it up. You'll spend a lot more time building, planning, and re-planning, but that's good. So many players complain about mid-late-game boredom when they've got dorms and farms and nothing to do. I say to keep improving, bit by bit, stepping on your own toes. Get each dwarf a private suite, preferably with their own study in a different room and a dedicated closet (with magma-washer for when the closets get overfull!). More importantly, though, fit individual rooms and studies into your existing fort by repaving and repurposing. While it may be possible, and simpler, to simple deconstruct whole blocks and rebuild them to need, it ends up much more lively when you do the bare minimum deconstruction to get something used for a new purpose.