Worldbuilding is about telling stories. Storytelling and worldbuilding flow from the same spring. When no one knew what lay on the far side of the hills or across the wide river, any story about those places was set in an imaginary land that could be as fanciful as the storyteller cared to make it. (“Snakes there have two heads, fish speak in riddles, and the people walk on their hands! I have seen these things, and I tell you they are true!”)
When beginning to sketch out a new world, the first question I ask is not about cultures, races, geography, politics, science, or gods. All of those come later. Question #1 is, “What happens here?”
If your imaginary world will be the setting for a novel or short fiction, then the story is clearly the motivator for most world design decisions. If you’re building a world as background for a roleplaying game, however, then the world probably won’t be oriented around a single defining event or plot. Instead, you must ask what characters will do in that setting. What types of escapades will heroes be drawn into? Will their top concern be staying alive in a world of horror, becoming rich through guile and martial skill, exploring the unknown, reclaiming the wilderness, overthrowing a tyrant, staving off evil, or saving the world itself from destruction? What type of game/story will this world facilitate?
So ask yourself, what will adventurers do in this world of yours? Once you have an answer, you can tailor elements of the setting to fuel those types of adventures. For example:
Characters will explore dungeons, kill monsters, and take their stuff. You’ll need to seed the world with dungeons. Then put some thought into why all those dungeons exist and sprinkle the world with legends and clues that can lead characters to these lost and lonely subterranean mazes. Forget about complex trade connections, dynastic trees, and religious strife. They can come later, if at all.
Characters will walk from place to place, meet people, get into adventures. You should begin by watching as many episodes of Kung Fu as possible, if you don’t already have them all committed to memory. Then approach your setting in terms of sets and episodes. An episodic campaign doesn’t demand a high degree of continuity from place to place, but it does need plenty of small locales where interesting problems can arise.
Characters will carve their own strongholds from the wilderness and push back the limits of darkness in the world. This begs for a points-of-light setup. Since characters will begin at just one point of light, you can focus your effort there and leave everything beyond the confines of the heroes’ small world literally unexplored by them or you until a need for it arises. The world can evolve as it’s revealed.
Characters will be central figures in a world-shaking calamity that threatens all existence. You set yourself a challenge when you adopt this structure. Such a story is most effective when players care about the world. Otherwise, they’ll either go through the well-rehearsed motions of saving it pro forma because that’s what’s expected of them, or their attention will wander to other, more attractive stories like killing monsters or walking the earth. For the world-saving epic to work, the world needs to be extensive, the players need to have seen a lot of it, and they need to care what happens to it. You need to know your players, design an extensive world that catches their hearts, and be ready and willing to manipulate their emotions by threatening what they care about.
The purpose of clearly answering the question “what happens here?” is that it helps to eliminate awkward find-the-fun moments when players are between adventures and don’t know where to turn for the next one. When all else fails, a well-themed world itself can provide the signpost.