Anyways, I'm picturing something like Water World meets Road Warrior meets the Time Machine, except possibly better than that sounds, with some Teenage Mutant Ninja Turtles style animal-men (like Goatmen and Spider-Men and Snail-Men) and, essentially, whatever other craziness sounds good to graft onto it.
Maybe mash some alien invasions, communist scare stuff (A la Red Dawn), and we can paint a picture of a mankind that didn't get to the moon and halt, but managed to expand our empires onwards into space. The Cold War didn't quite end yet, so it was Capitalism on one side of the moon, and Communism on the other, and it was the spaceship battles that doomed mankind. Initially, it was small, crude spaceships battling each other with whatever weapons the scientists could devise- initially "space-versions" of conventional weapons, designed to work even in the vacuum of space, but gradually turning into bizarre nuclear-powered devices, such as the death-rays and heat rays and crude particle accelerators.
Hell, we could even say that the ever-increasing size of the spaceships is what doomed humanity, since the blasted bits would no longer burn up on re-entry, but would rather slam into earth and spread radioactive waste across the ground and into the water, poisoning men and destroying forests and crushing cities. With enough people devastated and the governments nearly bankrupt from funding enormous, elaborate space battles, anarchy would ensue and good old-fashioned lawlessness on a global scale would complete the catastrophe.
|Road Warriors everywhere rejoiced.|
Or something. This has all been a gigantic brainstorm, and we'll see what sticks.