Open world in Diablo 4

What does open world mean?

Blizzard says Diablo 4's open world concept is the biggest change they've ever made to the Diablo series. They define it as: seeing random players while exploring Sanctuary [1], and the ability to partake in activities with them:

Number of players seen

Outside of a Region's Capital

Like past Diablo games, Blizzard wanted players in Diablo 4 to get a sense of danger and remoteness while exploring much of Sanctuary. Also, because the game's world map size is 10-20x larger than those of previous Diablo games, other player sightings should be sparse. During Diablo 4's development, Blizzard once said it was inserting only 6-8 random players in a player's current Region, but this may have been tweaked upwards for release.

Inside a Region's Capital

Similar to their real-life counterparts, Diablo 4 Capitals, like Kyovashad in the Fractured Peaks, are meant to have a bustling feel. Players will come into contact with many other players here, especially at the Stash, Vendor, and Waypoint areas of town. Blizzard hasn't said the max number of players inserted into a Capital at once, but it's likely between 20-30.

Are Dungeons and Strongholds part of the open world?

No, Dungeons and Strongholds are private to the player (and their Party, if they're in one).

Completed Strongholds exception

The only caveat is when a Stronghold is cleared by the player, it then becomes part of the open world and can be visited by random players.

Open world activities

The whole purpose of an open world is for players to collaborate with each other to create spontaneous, fun moments. In addition to lots of smaller interactions, like chats, Parties, and trades, Diablo 4 offers the following open world activities:

Local Events

While out exploring, players will occasionally encounter random events with brief storylines, called Local Events. These events have 2 objective levels to complete, and are one of the only sources of Murmuring Obols currency, used for gambling at the Purveyor of Curiosities. Blizzard may boost the chance of seeing other players around Events, though this is speculation.

PvP

PvP in Diablo 4 is performed in a special area called the Fields of Hatred, which anyone from the open world can enter and test their skills. Top PvP players will achieve Hatred's Chosen status after besting many opponents, which puts a target on their back (like when someone captures the flag in Capture the Flag). Also, harkening back to the days of Diablo 2, players can take the ears of opponents as a souvenir.

World Bosses

World Bosses are massive creatures that are so big, the camera needs to pan out. They spawn a few times a day in special areas, like the Crucible in the Fractured Peaks, and have so much Life, that a dozen players are required to bring them down. This creates great camaraderie amongst the random players, and a sense of pride after a hard-fought victory.