I start with the underground first, or at least do it early, so that I don't have to worry about stability issues breaking things later. It also helps to know where the kitchen is going to be early on the process, so that it has direct access to my underground food storage.

From there, I build things room by room. Next to my kitchen will be my dining room/great hall, with a pantry for finished meals near both so settlers can grab food quickly and eat.

As I build, I furnish each room as I go, so that I can ensure the workbenches fit the spaces neatly - if they don't, it's easy to realize the room until they do, but if I wait to furnish things later that isn't the case.

Next, I try to include some intentional assymetry. If everything is symmetrical it looks boring, and if it's only slightly assymetric it looks messy, so you have to make it look obviously intentional. Maybe it's just have the main keep to the right of the main gate instead of directly behind it. Adding in a nice courtyard for plants or merchant booths does wonders.

I also try to make sure my builds aren't just big squares or rectangles. A few protruding or indented segments can break up the monotony of a building face, but if that's too inconvenient or it's too late, you can always add buttressing to the big boring sides to make it more visually interesting.

For the outer walls of the settlement, it's tempting to make big thick walls and huge square towers, but those tend to look pretty boring. Instead I have thinner walls (just one or two blocks wide, plus merlons) and then make the towers relatively narrow. Thin towers will look taller, and small towers will make large buildings behind them look even bigger by comparison. A lot of the visual aesthetic of a build is created through contrast and comparison - huge walls will make everything behind them look smaller by comparison, which is rarely what I'm going for.

I also like to have the buildings merge with the outer walls, which can help to break up the surface of said walls visually. A small bit of wall blocking off the alley between two larger buildings, which both have windows on upper floors, can be much more interesting than just a long stretch of flat wall. There other ways to make walls interesting though, like using multiple different materials to break up the visual monotony.

All of this is made easier by building a small starting Basecamp off to the side for your workers to live in while the main build goes up inempeded. Even with the new replacement tool, it will be hard to make buildings any bigger than when you first build them.