The Meaning of Life, the Universe, and Everything.
So, recently I have gotten bored with minecraft, and just wander around aimlessly, with no real goal. I just recently started fighting zombies in mass numbers, and skeletons, just because. Which gave me an idea.
The short version: As you build, you attract attention of unwanted visitors, and must fend them off.
The long Version:
The larger your buildings, and the more buildings you have, the more attention you will attract from mobs.
A smal house made out of just wood would attract a few zombies, and skeletons at night.
But, as it gets larger, you will start attracting creepers, and spiders. The larger the building, and the more buildings you have, the more you attract.
As you attract them, you can start attracting NPCs to live in your village. They can help gather materials, or fend off the mobs that are attacking.
It can be large scale, or small scale, depends on what the players builds, or what setting it is set on.