My kids are obsessed with Lego. We tend to buy a combination of sets (mostly Ninjago, Disney Princess, and City themed) as well as acquire boxes of random second-hand Lego.
How they tend to build is by “Legoforming” parts of our living space. My son will declare a particular area to be a certain “biome” (city, desert, lunar landscape, whatever) and they will start placing structures, fauna, utilities, and so on.