I try to start with a theme for the base. Sometimes I know ahead of time and look for a likely spot and others I just wander till the terrain tells me I've found the spot. Then I work to that goal. The power and machines and infrastructure just empower me to attain that goal. Once I got the base the way I like it and feel it is finished, I tend to get bored. But usually my goals are fairly lofty, so takes a while. I've hollowed out a mountain island and had a massive tower on top, I had a huge farm with a farm house, barn, windmill, sawmill, etc spread out over a good amount of chunks. Built in a high cliff box canyon up into the walls with bridges spanning back and forth at different levels.
Currently, working on a city. Trying to have each building have a function and sprinkled around just normal houses for looks. Gardens, roads, the whole thing surrounded by walls and towers. Still working on the palace, which is taking far longer than I expected... The idea is if you want to do something you have to go to the appropriate building. Walk down to the smeltery building, then over to the alchemist for some potions. Maybe stop by the tavern or marketplace to restock some food and then head over to the Arboritum to watch the butterflies or the library to make a new mystcraft age. Will have villagers milling about and some animals to bring it alive. That's the plan anyway. Bored? Not anytime soon. I shudder to think what I will do next update as I seem to esclate the scale each time. Maybe a battleship. Oooh, maybe a battlestar! That would be challanging.