I've been thinking about this since I first started playing Dark Heresy a couple of years back:
Alright, let's imagine that at the end of a Black Crusade campaign the Corpse-Emperor is destroyed and Terra gets absolutely sacked. The Imperium is defeated and humanity lies, once again, scattered across the stars while the xenos and the dark forces of the Warp ravage the galaxy.
Now then, what would the galaxy be like, and what would a campaign set in the post-Imperial galaxy be like, whether the players are heretics or not?