- corporations take over
- crime takes over (might be same as above)
- science run amok (e.x. killer animals, robots, or diseases)
- post-thermonuclear war
- government run amok
- religion takes over or is abolished completely (depends on who is writing)

any others?

on topic, it depends on the vibe being looked for - some are horrific (zombies and radiation victims) some are kinda played humorously (ie - robocop)