For my campaigns I prefer complex storylines with lots of twists and mystery. The strategy I outline below can be adapted to a large-scale world building style game or a small self-contained 1-2 month campaign. It's adaptable since you build up complexity as you go. Lastly, this approach allows you to build your world and your story simultaneously which, in my opinion, produces more organic and believable environments.
A good, believable world or setting relies primarily on the fact that our universe is a deterministic place - things never happen isolation and events always influence other events. I imagine the worlds I build as a massive network of interacting events and use this to make the world more believable.
The following is an example for a much smaller-scale world. This is a rather mundane example but it's used for the purpose of demonstration. I assume that you already have a sense of player motivations/backstories. Take out a large piece of paper or poster board and start writing:
- Begin by writing down some ideas/words about the kind of story you want to tell. Perhaps yours is a political story where the party unravels a political plot. Maybe it's more of an adventuring plot where there are strange events happening in a city and you need to find the cause of it. Maybe it's more of an exploration-driven plot where the players need to wander around the world picking up clues before they realize they are part of a larger story. Maybe it's a background-driven plot where you integrate their backstories together and they realize that all of their backstories relate to a common source.
- Draw a box on the paper and write the following in it: Begin by thinking about a setting/circumstance in which you would like the characters to meet. Of course, this might be difficult since you don't have a story yet but this is just to get it started; you can make modifications to this later. For our purposes, let's assume the setting is a small city.
- Draw an arrow from the above description to a new box which describes the condition the city is in. I usually write "condition" near the arrow. Let's say that the city guard has recently dwindled in numbers.
- Draw an arrow from here to a new box which describes the cause in the town that may have caused this problem. Perhaps a number of dangerous monsters have increased in numbers outside the city and they are killing the town guards.
- Draw a new area to this box describing the cause of this event. Maybe a group of Kuo-Toa were normally hunting the monsters in the swamps outside the city but for some reason they have vanished.
- Draw a new cause to this event. Let's say the Kuo-Toa have been using a special substance in their rituals that has caught the interest of a cult who is trying to open a portal to the Elemental Chaos to let in demons. They have captured the Kuo-Toa and keeping them in a cave system that lies in the mountains outside of the city.
- Now draw some arrows from the town. What suspicious conditions might be present in the city? Perhaps the king/town leader is acting strangely, perhaps there is an influx of a certain group of new people/cultures, perhaps people are dying strangely. Let's say that the town leader is acting a bit strange and people seem to be dying.
- Now let's link this back to something we've already created. Maybe the cult needs something for their rituals that's found in the caves and their mining process has introduced metals into the ground water. Furthermore, let's say the city leader is involved with the cult, perhaps with the motivation of something that he desperately needs.
You can keep going like this, connecting cause and effect chains to create a believable scenario. Whenever you do this, think about what the repercussions of each event might be and how it would effect the setting. As you build up the world more and more, you can start adding more specific elements like city culture, language, religion, and government. You can think about the mood: what would the attitude of the town be? Fear? Stress? Panic? I write these kinds of words under all of the arrows and boxes where appropriate.
Next, think of some NPCs and add them into the mix. Who might they meet in this city that is related to the events you have outlined above? Will the be friendly to the characters? Will they try to mislead them?
I find that when I work in this way, it's fairly easy to come up with a complicated world with lot's of causal events that are linked together.
Now the final step is to determine player goals and sequence of events. In your mind, the events above will probably be very linear since you know the story. That means that you need to pick events that can happen based on what you've outlined in a way that is is out-of-order. Without all the information at their disposal, there is a sense of tension and mystery for the players and it makes it much easier for you to mislead them and create plot twists. They will come to erroneous conclusions because of how you have limited them. These steps are kind of "filling in the blanks" - you can figure out what role they will play in the story and what order they will find out the information in.
This step allows you to create an immerse world to the players because they are being shown the reaction to current events happening around them without understanding the context of these events. That's their job - to unravel this web and see how it all fits together. The more tangled you make it, the more complex the story becomes.
Examples of reactions:
- They city guards may be anxious or nervous, less in number
- Citizens may be sick
- Citizens are robbed and attacked by bandits outside the city because the town guard has dwindled in number.
- Citizens complain the town leader is acting strange
Examples of goals:
- Player needs to visit the town leader
- Player needs to visit an apothecary who can tell them what these symptoms mean. This may lead them to some mining systems within the caves outside the city.
- Players need to ask town guards when these events happened. Might lead them to a house in the city with suspicious activity. This might give them clues about what is going on in the caves outside the city. Maybe it will lead them to the swamps where they can get more clues.
You can then leave it open how the players investigate these things. Always think of 2-3 different ways they can approach any goal so they don't feel railroaded. It's even better if the goals are non-dependent. If they can do them in different orders it gives the impression of a sandbox kind of environment.
This is just a simple example. You can do the same sort of thing for cities in a continent that have economic and political relations if you are going larger/epic scale. You will have much more at your disposal this way (religion, xenophobia, social disagreements etc) that can affect the way these cities act with each other. In this case, I suggest dividing up the campaign into self-contained acts with information that carries over between different acts.
It's all up to you how complex you want to make it but I think this approach works well for creating a believable world for the players filled with mystery and a touch of sandbox.