There are a lot of open-world video games. They struggle with the concept, because video games have to be pre-built and therefore are much better suited to structured adventuring. The result is that many are carefully constructed to drive the player in a particular direction to experience the world the way the designers planned. They may present challenges too tough to handle, badger you with offers to go to a different place, or make a certain place the only interesting thing you can see on the horizon.

So how do you structure an open world? The key is meaningful choice and layered encounters.

Meaningful Choice

In a TTRPG, the GM has the luxury of creating new content between sessions. So you can give the players options of where to go and what to do and develop the content just ahead of their going there and doing it.

For example, they start in a town or small city on the borderlands and have the choice of going into the great forest, the hills, or to the big city on the coast. Each of these needs to have a reason the players would want to go there: specific challenges and treasures that are to be had there.

It’s important that the type of monsters and dungeons to be found in the great forest be different from those in the hills. If there are mainly orcs in both places, it’s not a meaningful choice. As the heroes advance in level, their travel should be broader, to the point that they’re not just choosing between different points of interest but between different points of interest in different realms.

It’s also important that the encounters be interesting. It’s not much fun exploring the Barrow Mound Moors instead of the Orcish Wilds if both just present a series of combats. Instead, the encounters need to be different, the treasures different, and the secrets uncovered different. After all, there’s no point in exploring anything if there are no secrets to be learned about the place.

Layered Encounters

It’s also important that there be known challenges too great for the heroes to handle yet. There may be a vampire’s fortress in the great forest, giants in the hills, a dragon in the mountains, and a murderous thieves guild in the city. This is the concept of layered encounters.

It’s likely that the heroes will come across orcs in the forest, but they should know it’s possible for them to come across the vampire’s lair. It’s all a matter of what hexes they venture into. At low levels, the same can even be true of an individual dungeon; if the heroes have an inkling of what they may encounter there (orcs and ogres living in the same caverns), they can steer clear of an encounter that’s too powerful for them–unless they come up with a particularly good plan for how to deal with it.

Putting Them Together

You can put these two concepts together by mapping some known points of interest that are perils (of a known level) and rolling up unmapped points of interest, like orc tribes, as the heroes explore the map. After all, any road trip will result in some sight-seeing along the way that may end up to be just as memorable as the ultimate destination.

If you want to offer the players some overarching story, that too needs to be done as a set of options they can pursue, so that the ultimate campaign is as much their doing as yours. After all, you likely have multiple ideas for what the heroes could do; why not ask them? Inventing one story and expecting the players to buy into it and play along is asking them to fulfill your one specific fantasy rather than all players’ (including yours) collective fantasy.

Besides, what emerges as the campaign is probably better than any specific thing you’d plan. Letting the players gravitate toward one NPC that you then leverage for a storyline will be much more effective than hoping they’ll buy into an NPC you plop in front of them.

The best way to do this is to have each option likely reveal some nefarious scheme or secret the bad guys are sitting on. For example:

• Orcs dominate the leafy wilderness (altho other forest creatures can be encountered there, such as owlbears, ogres, and fairies). They often attack trade caravans in hopes of carrying off trade goods to use and captives to eat. But they’re secretly allied with giants who have a plan to seize an entire human town soon. Reporting that fact would put the party in good standing with the local lord.
• Goblins dominate the evergreen wooded hills (altho other hill creatures can be encountered there, such as trolls, dire wolves, and barrow wights). They prefer to attack individual or small groups of good folk as captives to eat, because trade caravans are defended. But they’re secretly occupying an abandoned church of good alignment that retains some of its sacred objects. Recovering those objects would put the party in good standing with their church.
• Lizardfolk dominate the dense swamp (altho other swamp creatures can be encountered there, such as giant snakes, crocodiles, and shambling mounds). They attack good folk on the roads as captives to sacrifice to their evil god. But they’re secretly working with an evil witch who is leading them in building a temple to their evil god. Bringing this to the attention of the local lord and church would be greatly appreciated.

Secrets

Roll 1d6 for the type of secret the bad guys have and 1d6 for the nature of that secret.

1. They work with… (1d6: 1=other similar bad guys; 2=an evil cult; 3=a thieves guild; 4=a witch or hag; 5=dark elves; 6=bandits).
2. They work for… (1d6: 1=giants; 2=a dragon; 3=a necromancer; 4=a rival lord; 5=a vampire; 6=a menace from the deep caverns).
3. They occupy… (1d6: 1=an abandoned temple of good; 2=a cavern complex that connects to another near a settlement; 3=a seized manor; 4=an ancient temple of evil; 5=a crypt of awakened undead; 6=a lost ruins of historical interest).
4. They’re sitting on a cache of… (1d6: 1=maps; 2=sage chronicles; 3=documents about puzzles/traps in another dungeon; 4=clues together with a mysterious key; 5=lost relics; 6=lost magic).
5. They’re building… (1d6: 1=an evil temple; 2=a fortress of their own; 3=a portal; 4=a watchtower; 5=a bridge across a river to ease their raids; 6=a big trap).
6. They’re planning… (1d6: 1=to attack a trade caravan; 2=to raid/capture a settlement; 3=to sacrifice captives; 4=to cause environmental devastation; 5=to destroy a good church; 6=to disrupt trade/farming).

Follow-on Effects

Sometimes, these secrets can lead directly to another adventure. Sometimes, they can end with the heroes getting a dinner in their honor that gives them contacts they can use later. Sometimes, they can end with the heroes getting gifted a modest magic item or a knighthood. All these enrich the campaign in a way that a monetary payment doesn’t.