1. The dead rise from their graves and feast on the living, gaining more numbers with every infection.
2. Aliens invade, weilding technology beyond our comprehension and with an intent to harvest.
3. Dinosaurs return and once again roam the Earth... Origin unknown, but perhaps a cloning expiriment gone wrong?
And then there we are: Mankind. Separate regimes pulling together with hopes to survive and even fight back, but do we have the means to?
I hope this opens up some interesting and hilarious discussion, please feel free to contribute your theories!