5th ed DnD, The Ruins of Baldur's Gate

Could turn out to be a horrible idea, but: So the game revolves around the city, and the players have a camp somewhere in or around it that serves as their place of respite and planning. The party is under the employ of a faction that runs that camp. Since the location is rather static, any extra players can make a character that is located at camp, and pretty much can RP and whatnot on a camp thread with eachother there if they so please, and interact with the party when they visit camp. Any additions to the party are handled IC by the players, who can add you on, trade spots, or whatever- just sounded more fun to have an IC "reserve" list than an OOC one, but totally optional.

One player even made a character that doesn't seem to intend to join the party but rather works at the camp in support of the faction that is sending the PC party out into the city.

TL;DR: Making it up as we go.