I'm no mega-projector but I do do some things to make my forts interesting. From a visual aethetics point of view I like to give the fort's rooms some kind of shape not entirely related to function: for instance, giving the entrance hall a big oval room at the end of it. It gives a sense of grandeur quite apart from the stark utilitarianism of a bunch of rectangular spaces for dwarves to do things in. Like this:
As time passed I found the rock surrounding the oval was a convenient area to put offices of nobles in. 1 tile would serve as a diagonal passage into a rectangular room, with the interior walls engraved without changing the appearance of the main room.
Another thing I like to do is put interesting shapes to use in some way. This screenshot depicts a magma death chamber flanked by two long hallways. It was originally intended to be just plain hallways zig-zagging, but then I realized I could do more:
To make them I took a screenshot and measured the proportions of the actual "statue" character from the game itself and designated an equivalent area to be mined out (e.g. a 3x2 pixel box is translated into a 12x8 tile area to be mined out in-game).
Large-scale goblin traps are of course particularly engrossing.
Further consideration for aesthetic structure building is to think of something that would look cool and just start building it. In the natural course of events something memorable will happen on or near it. I built this bridge over a very wide river/waterfall, complete with a "gatehouse" on one end of it, and it became the site of a terrible battle:
After the battle a couple of dwarves were surrounded by macegoblins on the side with the gatehouse and
literally beaten for months.
Finally, as a somewhat non-visual aesthetic consideration, I like to plan my forts with a visit by an adventurer in mind. I like to set up interestingly shaped burial areas for all the dwarves, memorialize every dwarf and place the memorials next to their coffins, and pay special attention to documenting the fort's history in some way. Here's an example of this, post fort-destruction:
It's not visible in this image because it's a reclaim, but I also entombed dead goblins in the wall in the entrance area where the "mosaic" is (made of fireclay and orthoclase). The differently colored patched of wall mark where each space was carved out, a goblin's skeleton was dumped, and the space re-sealed. In front of each was a memorial to the goblin it contained. I find this memorialization of a fort's history to be the most meaningful exercise of all, and it certainly takes time and attention. As a part of this memorialization I also like to mint at least one set of coins every year when possible, as well as place statues in meaningful places depending on their subject.