Maybe you shouldn't have trashed that donation flyer for the Cargo Pilots Association, hmm?
IMO, this game doesn't necessarily need a traditional "end game" - like we have to solve the big Apocalypse Puzzle and Cure The Zombies or whatever. I think the "end game" here should be like it is in a repetitive puzzle game - the difficulty ramps up until it simply is not possible to survive another horde night. So the "end game" is "survive as long as possible" with the understanding that the game will eventually make it impossible to survive.
That can tie into "bosses" (which are...I guess...extra strong zombies? with special loot perhaps?) and exploration quests to, say, find ultra-rare weapons or traps. But weaving in a whole story where you have to defeat Boss A to find part of the story which leads you to Boss B and...and...and...finally Boss X is the "end" of the game? IMO that doesn't fit 7D2D.
Flesh out the questing, add more NPCs (bandits, etc.) to give flavor, but the goal of the game should always be survival. The vanilla game I mean. We should see tales of brave but futile attempts to survive the 10th horde, and the one player who made it to the 12th horde is seen as a hero.
I hereby respectfully disagree with all of you who will disagree with this, but I proactively understand your very valid points and differing opinion. Except for YOU. You know who you are.
Also, if the game does go into the full story-with-an-end direction, I ain't even going to be mad.