Yeah, story mode, Im already have ideas about two endings, with story already present and after completing main chain quest when you entering trader :
a) Good ending - Pathetic congrats text "blah blah you find survivors, you're best", brave music, and titles runs.
b) Bad ending - after entering trader doors locks behind you, and endless zombies spawns. After your death - sad music, message "blah blah no more hope etc" and titles runs.