Minecraft wiki entry for Building_a_metropolis, associated text: Void: Create a superflat world with the Void preset, then build a city just like you normally would, but with the buildings floating over the void. Due to the lack of natural resources in a Void superflat, this city must be built in Creative. A city like this will be a dangerous place to live in, as the risk of falling into the void is constantly present.
Wasteland: This is best to build in the badlands or desert biomes. Have something like a cowboy town, with a sheriff. You could add "tumbleweeds" (a bunch of dead bushes). To simulate pistols, you could give all the citizens crossbows with Quick Charge and Piercing. If anyone disobeys the rules, send cowboys on horses with armor (preferably leather horse armor). Make a huge herd of cows for the town's food and meat supply.