Every horde mode map could tell a little more about the main over arching story, it would give horde mode a greater purpose and gives the survival lore buffs a reason to play the mode.
Maybe even some horde mode maps could fill out some of the survivor npc’s back stories, for example if some trader/safe zone npcs remember/survived the outbreak there could be a horde mode map based on this and you would play that npc during that specific horde map and play out his/her memory and get some lore along the way.
Horde mode lore should relate to the over-arching lore (the survival mode lore)
Perhaps a horde mode on a city street, where there’s power and fires burning in the background?
And perhaps through it you would occasionally see a vehicle crash nearby outside, or vehicles driving by quickly, perhaps even a car crashes through an area after a certain number of rounds and you can go through the hole caused by it to get to another area?
I would prefer for there to not be any set lore for anything outside of the main game. It should be left to the mapmaker to create lore for things that wouldn’t normally have it. If we do not have a set story for Horde Mode, then anyone can come up with one and have it equally as good since it wouldn’t be comparing to the ‘actual’ lore.
I’d prefer that Horde mode maps are simply mutually exclusive scenarios that fit the theme of the map while having little connection to the main maps but this is the map maker’s choice. Though if official horde mode maps exists, Nelson might have NPCs in a survival map that references the horde mode map.
Same goes for Arena (Battle royale) and Escape (Stay alive or infect all).