I posted once before about creating a Western Marches style campaign. I never really went anywhere with it because I didn't have a theme beyond exploration.
My thought is that the players are, more or less, hired explorers to map and investigate a newly discovered land that seems to be void of "modern" civilization. Through exploration, the players uncover evidence of an unknown kingdom in which everyone appears to have simply vanished: cities are abandoned and crumbling, roads are overgrown, structures collapsed, etc. The previous inhabitants (good guy races—humans, elves, dwarves, etc.) are no where to be found, and the disappearance seems to have occurred within relatively recent (100-200 years) history. The traditional fantasy bad guys (orcs, goblins, Giants, etc.) have begun to spread throughout and occupy some of the structures that still stand. The intent would be that through exploration, the players would be able to piece together information about who "they" were and why they disappeared.
Where I fall is that I don't come up with good stories that don't feel contrived. I need help coming up with plausible suggestions on why this civilization vanished. Anyone have any fun suggestions?
My overarching theme is standard fantasy with a post-apocalyptic feel (DayZ, Walking Dead, etc. minus the hordes of zombies).