Co-op
The game's Co-Op Mode, open from Adventure Rank 16, lets up to four players share one host's world: exploring it together, clearing domains and bosses as a party, each player with one or two characters on the field. Around it sit the friends list it is joined through, visits to a friend's Serenitea Pot, Genius Invokation duels between players, and the Imaginarium Theater's Supporting Cast, characters friends lend each other.
Why deferred: the world keeps no server. Each player's progress is kept in their own browser, the daily reset is their own 04:00 (enemies), and the world is the person's own, which the Genshin proposal sets out of scope for multiplayer. Co-op needs a host's world held somewhere guests can reach it, the host's state authoritative over every guest's actions, and each action relayed between players as it happens: a server, and other players to meet on it.
Revisit when: an account keeps the player's world on the app's server rather than in the browser, so a host's world exists for a guest to join. The app's existing Web PubSub is then the first transport to price for relaying a party's actions.
Cheaper interim: none is needed. Every system the world proposes is built for one player, as the game itself plays outside co-op.
Sources
- Co-Op Mode, Genshin Impact Wiki: the unlock at rank 16, up to four players in a host's world and their characters each.
- Serenitea Pot, Genius Invokation TCG and Imaginarium Theater, Genshin Impact Wiki: visits to a friend's realm, duels between players, and the Supporting Cast lent by friends.