Not sure if this thread is still alive, but i am also sucker for defined ending. From my point of view, you are left there because of debt, so there are people living okey-ish somewhere. So why not have ending of leaving that hellhole, like in Conan Exiles - you do you, until you got enough, then do some minor steps and gate endgame cutscene of leaving.
My idea was reach max rep with traders and take reward:
1. (explorer type) get map to some buried POI like metro station, find location, dig into it, clean it from zombies and sit in metro to leave;
2. (base builder style) get extraction beacon or recipe to build it, survive horde after activation until some fancy military helicopter takes you away.
I think second one would make some people compete in base designs and character builds, like "this built survived final horde", or "this base crashed in first wave"...