There are two things you need to consider when planning your fantasy world-building exercise and adventure campaign: the theme and the central tension. Daggerheart has gotten a lot of attention lately for this sort of thing, which it calls “campaign frames“.

Concept

The concept of the setting is the overall type of world, and feel and tone that comes from the cultures and creatures you populate it with.

Examples

• It’s a grim and dark world of mountains and broken lands, filled with perils that threaten the heroes’ bodies and immortal souls. It’s filled with wicked sorcerers, vampires, doppelgangers, spies, swamp witches, loathsome serpents, lycanthropes, demon-summoning cults, duplicitous NPCs, and intricate locks that hide dark secrets.
• It’s a world of grand adventure across open lands and primordial forests, with fantastical cities and awe-inspiring ruins. It has knights on griffons, unicorns, stately ancient temples, magnanimous nobles, orcs on worgs, mysterious societies protecting ancient secrets, dragons good and bad, bulettes, and puzzles that unlock huge doors.
• The realm is broken into a number of small islands, so travel is by ship. The world is full of human and gnoll pirates, sea serpents, merfolk, mysterious shipwrecks, dark storms, exotic cultures with colorful costumes, genies, animated objects, scheming rakshasa, liches in lost tombs, grandiose merchants, and shocking revelations.
• The realm is mostly arid plains populated with lions, elephants, and giraffes. But castles protect against the scourge of gnolls and ogres. Ancient pyramids and hidden tombs are ripe for looting but guarded by a sphinx or couatl and defended by mummies. There are people of every color and ancestry, scheming sorceresses, and giant snakes.
• A catastrophic earthquake wrecked the realm when the heroes were growing up, and now a lot of the old cities and castles lie in ruins. Now and then, there are aftershocks that cause more damage.
• It’s a mega-magic realm of huge, fantastical cities. There are waterfalls that go up, a city on the back of a gigantic tortoise, and a “library” where people in paintings teach students. It has flying ships, magic portals between important locations, dark crags haunted by beholders, magic weapons of legend, and ancient artifacts of untold power.

Central Tension

The central tension is usually the villain, but your campaign may not have a specific villain but rather a series of villains. Even so, there is likely something at the center of the setting that creates tension and generates conflicts, such as factions or rivalries, an evil overlord, or a deep mystery the heroes may one day find an answer to.

Examples

• The land is blighted, but the cause is uncertain. Shadowy factions vie for power. Elemental magic is at work in dark temples.
• A war of succession changed the ruling house, but powerful nobles have declared the king illegitimate and are preparing for war, while the king does the same. The heroes must choose a side.
• The heroes learn about ancient horrors in the depths of the deepest caverns and seek them out and slay them while trying to maintain their sanity.
• An evil overlord rules as a tyrant and sends minions into the heroes’ lands to spy and weaken the society. Certain monsters do his bidding, and faithless NPCs abound.
• The emperor wields power capriciously from his distant palace, and his troops and spies are everywhere. The heroes are wrongly accused and declared outlaws.
• The heroes are all rogues who plan and pull off heists against corrupt nobles and merchants and sell the stolen goods—or provide them to good organizations.
• Magic seems to be coming to an end. Anti-magic zones cause wizards to lose power, and spells sometimes fail under the best of conditions. It’s unclear why or if it can be stopped.
• An ancient dragon still plagues the land as it has for two centuries. Its lair is surrounded by the lairs of lesser monsters. Its treasure is legendary.

Special Rules

As noted in my post about combat-light campaigns, you may need extra rules particular to your setting, such as more robust and gamified negotiation rules, arbitrage rules, carousing rules, crafting rules, and battle of wits rules.