(takes a deep breath)
Global tensions brought on by increased populations and decreasing resources will spark global conflicts within unstable regions resulting in increased arms developments. In the race to create the perfect weapon without the destruction of natural resources, the zombie virus will be created and used, but grow out of control. To erase the virus, the world government will launch nuclear missiles to kill the undead hoards, but the resulting fallout will tear a hole in the fabric of space and time. This rip in reality will bring about all the mythological and religeous "end-of-days" scenarios, but these gods and deities will be wiped out by the coming aliens from another dimension, who seek to convert the godlike power into an energy source and morph what is left of humanity into repurposed slaves. However, these aliens will awaken the Old Gods, who will emerge from our oceans and fight with both the mythological deities and intergalactic aliens, with humanity struggling to survive within their changing planet. Unfortunately, the rip in space and time will continue to grow, causing our sun to go supernova and a blackhole to form, what we know of existence will collapse onto itself...
...and then like all things, the universe will come full circle, and the end will once again start at the beggining.
...and just for fun, throw in something about robots and global warming.