I once started a story where the two sides in the Final World War had created automated killer satellites. The satellites worked by detecting characteristic human brainwaves and nuking the DNA in the brain stem with entrained radiation beams.
There's a little bit of a plot hole about people who hid in bunkers and so on, but people didn't know they'd been hit and died some time later, so there's a bit of wiggle room over it.
Of course scientists and military were the first targets, so it didn't take long for the satellites to run out of control and over time kill all the humans on earth, except... some genetically engineered dickgirls :) Their brainwaves are just different enough.
So the girls have all the infrastructure intact, they're genetic scientists, and they resolve to rebuild mankind in their own image - unaggressive, empathetic, and happy to pass the time with sex instead of conquering everything. Oh and they don't age.
It was basically a morality story, about homo sapiens as a plague species, too aggressive to survive; and maybe getting into evolution as a system.
My very moderate storytelling ability tragically failed the test of turning this into a gripping tale, so if anyone feels like using any of it, please help yourself. Drop me a line so I know to read it.