Here's a short, rough version of my idea:
The Four Horsemen of the Apocalypse have been watching over mankind for millennia, growing tired of their slow decay and rising population. So the Four have come up with the idea of a tournament of death. Each will pick a group of humans to fight for them, bring them to a different dimension, kill them and reincarnate them. During their re-birth, they receive powers or skills to use to fight their opponents. Whichever Horseman wins the tournament, they do to the human race whatever they please. War: a war that will kill all humans and no nation stands victorious. Famine: to have every human die slowly of hunger. Pestilence: to plague humans with fatal, incurable diseases. And Death: to enslave the human race, only to slaughter them all in the end.
Anyone interested?