There's nothing wrong with starting rectangular, andrea. It's hard to concentrate on making artistic construction when one has to stop every few months and deal with caravans, ambushers, seiges, and migrants. Something I realized after reading a lot of forum threads is that some of the people building mega-projects have turned off invasions, severely limited migrants, and possibly turned off the weather, so that they have time to get the layout started well before taking on the rest of the game. There is nothing wrong with what they have done, but we should not berate ourselves for making great creations more slowly while dealing with all the distractions from the very beginning and ending up with 200+ Dwarves with 16 FPS.
My own fortress entrances tended to start as burrows into the sidehill, and then evolved into square courtyards on flat ground. Now I am considering combining the two into something more like a Dwarven mountain fortress.
My underground layout started as snaky 1-tile-wide tunnels with rooms where-ever they fit, and then evolved into strictly regulated orderly grids of wider tunnels and 11x11 rooms (subdivided into 5x5 for bedroom areas and combined for larger areas such as the dining room). Recently I have loosened my grip on the layout and started allowing my plans to adapt to the terrain instead of forcing a square grid on everything. One thing that changed dramatically was my sleeping areas. Since Dwarves don't care about privacy and the walls/doors don't reduce noise, I have been giving the commoners 5x5 bedrooms in larger open areas. The only reason for walls is to hold engravings, and they don't need those in their bedrooms. They have the Royal Dining Room to supply that. In return, they now all get cabinets because I don't have to make all those doors.
My fortresses that are still using the square courtyard entrance design have been learning to make pyramids overhead. I have also begun making circular sunken statue gardens for outdoor meeting areas, with designs in the tiled floors. For instance, Firechannels, founded by The Tufted Cats, has a floor design of the head of a cat with tufted ears. These are proceeding quite slowly because of all the other distractions. Firechannels has the pyramid only half done and the meeting area about 80% tiled, and the king has already arrived. There is a nice obsidian-block road and a soap-paved brookfront, though. I like to think of the Dwarves going out to bathe in the brook, pre-soaped by their walk to the water.
Edit: Here's Firechannels in Year 11. The fortress is centered around a 6x6 shaft down the middle, centered under the pyramid, and 6-tile-wide corridors from that shaft. There are four 15x15 rooms centered around the shaft for workshops and stockpiles until the dining/sleeping levels are reached. Then the rooms are much larger for a couple of levels. Below that is the region of the nobles and royalty, who have engraved walls. Farther from the shaft are oddly shaped areas such as the archery ranges, the obsidian experimental area, the forges, the shopping mall, and the three circular meeting rooms. One is the outdoor cat-head statue garden visible beyond the pyramid. One is an emergency underground statue garden, for use during lockdowns. One is a now-defunct waterfall garden that had to be abandoned because the annual freezing of the waterfall mist killed Dwarves. That is barely visible on the lower edge. So, my designs have lossened up considerably from the rigid mathematically determined repeating square grids that I used for months, but I am not yet to the stage of fluid designs which adapt best to the terrain. I have learned a lot about what the Dwarves need, and I am about to add some tougher opponents to give me a chance to develop more complicated defensive strategies.
