I am intending to run an open world campaign at some point, and I am wondering how to implement it. The objective is to, rather than having a plot, the characters get to travel around, taking quests, choosing to loot dungeons and other locations, and instigating their own action. The world is real, however, and things have consequences. Party is asked to stop hobgoblins from preparing for war, and refuse, so those same hobgoblins attack an in-game month later. Other adventuring parties might also be roaming around, capable of snatching up quests, aggravating player character, teaming up, or saving the city from getting obliterated because the party was off selling gallons of mayonnaise to bullywugs, rather than stopping a dragon. (If I'm capable of enough players and DMs, they might even be controlled by various other tables in the room, updating a log of what has been done that could affect the PC in real time, so three parties don't raid the same dungeon and each take all the treasure in it. Of course, this kind of undertaking would require recruitment that I'm not capable of) Only one problem. I don't know how to plan this open world. How do I create a region with cities and towns, containing NPCs with predetermined quests that the party could take, as well as possible adventure locations, wilderness areas, and domains of monsters? To make it even more complicated, I might have to come up with a general time-line, in order to show what would happen without the meddling of the players, what the meddling would affect, and what could be meddled with at, say, two weeks after in-game start. I would really appreciate any advice people have. 

I'd definitely recommend "having a plot". It is much easier to organise a living world when you have an overarching plot everything is relative to and impacted by. You don't have to force it on your players or even reveal it, but simply knowing that something on a larger scale is happening somewhere in the world makes it simpler to come up with small scale events that fit into the world's context.

If you know the bbeg is plotting to bring about a zombie apocalypse, perhaps the cities in the vicinity have frequent reports of silhouettes moving about the graveyards at night. Maybe there are rumours of cults practicing unholy magic among the nobility. Or maybe crops have been rotting for no apparent reason. You don't have to spell out the overarching plot for the characters or even the players, but knowing the reason why things are happening the way they are makes it much easier to tie everything together.

As for how you create content for your world, there are two main paths: come up with it yourself or use already existing content. Coming up with original content is time-consuming and it can be very difficult to be consistent and make everything fit together. But it is a very rewarding practice. Using already existing content is much faster and most likely fit together already (if taken from a single source). And there is nothing wrong about repurposing great content. If there is a book, movie, or game you are absolutely crazy about, bring it into your world! Places, people, stories (quests), etc. Take whatever you want and give it your own touch! Most DMs I know, myself included, take inspiration from some source(s) they find awesome, and build upon it to create their own unique world. 

Start small with a local area in which players can hear rumors of the wider world (so you can shape it before committing it fully to paper), populate it with a few interesting features for the players to explore and make the world feel more real. 

Then, I include about 3-4 main quest hooks that lead out in different directions out into the world. For session 1, I plan out about 1 encounter that goes with each quest hook, that way you have something prepared no matter which way they go, AND you don't have to worry about wasted effort on the quests the players didn't go with. Then next session (since letting the players get to know the town, listen to hints about the wider world, meet a few NPC's that indirectly represent various factions, get the quest hooks, choose one, and then do the first encounter, will probably be a whole session) you can have the rest of that quest prepped, and you can map out the region that it focuses on, that way you're expanding the map as the players are traveling it. 

And I typically don't show my players my maps, that way I'm free to change stuff as I see the need. Maps were rare throughout a lot of history, so to get to know the world they actually have to keep track of names and places themselves which can be more immersive than just looking at dots and names on a map. 

Hope that helps. 

Quote from BeyondMisty >>

I'd definitely recommend "having a plot". It is much easier to organise a living world when you have an overarching plot everything is relative to and impacted by. You don't have to force it on your players or even reveal it, but simply knowing that something on a larger scale is happening somewhere in the world makes it simpler to come up with small scale events that fit into the world's context.

If you know the bbeg is plotting to bring about a zombie apocalypse, perhaps the cities in the vicinity have frequent reports of silhouettes moving about the graveyards at night. Maybe there are rumours of cults practicing unholy magic among the nobility. Or maybe crops have been rotting for no apparent reason. You don't have to spell out the overarching plot for the characters or even the players, but knowing the reason why things are happening the way they are makes it much easier to tie everything together.

As for how you create content for your world, there are two main paths: come up with it yourself or use already existing content. Coming up with original content is time-consuming and it can be very difficult to be consistent and make everything fit together. But it is a very rewarding practice. Using already existing content is much faster and most likely fit together already (if taken from a single source). And there is nothing wrong about repurposing great content. If there is a book, movie, or game you are absolutely crazy about, bring it into your world! Places, people, stories (quests), etc. Take whatever you want and give it your own touch! Most DMs I know, myself included, take inspiration from some source(s) they find awesome, and build upon it to create their own unique world. 

 As far as having a plot, I do think this helps, but you don't need to stop yourselves at just one. My open world game has about 3 or 4 plots, between the dragon worshipping cult operating in secret across the land, the upcoming war between two baronies, the wizard to the south and his army of goblin mercenaries taking over the region, and the rogue necromancer trying to become immortal.

I have a page of notes tracking their various schemes step by step (not planning too far ahead since plans should change when players become involved), that way you can see the impact they're having on the world as they go unless someone steps up. 

The plans don't all need to be super twisty and intricate. Some villains are just after power and use brute force to achieve it.  As long as they feel distinct and there's someone interesting about each one for the players to get into, the fact that multiple villains are vying for control of your world/country/area, will make it feel more real, intricate, and urgent. 

by "plot", consider mainly the reasons for staying in the area - why don't your adventurers head off to yonder mountains in search for adventure?

finding something big to tie together (an intrigue plot, a criminal mastermind, a web of lies, a necromancer, etc.) will keep the players in the same place, uncovering more and more until the whole "plot" has been revealed and they unlock the boss fight. Making this plot something which affects the location (it's slowly sinking, or everyone starts dying, etc.) will give the party good cause to get involved with it eventually, even if they go off side-questing and such for the start of the game! It will also help keep them engaged if they feel like they are a part of something bigger!