I am thinking of doing a nation RP in the Warhammer 40,000 universe.  

The basic setup to allow this to happen is that the largest tyranid hive fleet to ever be encountered has at last reached the fringes of the Imperium and is beginning to carve a swathe through known space with nothing capable of even slowing it down.  Everything is thrown into prayers to the Emperor sacrifices growing a hundred fold in his name.  In an act of desperation a radical Inquisitorial group launches an attack and assassinate the Emperor to free his spirit at last.  With a psychic blast so powerful that killed millions just from sensing it the tyranid hive fleet was annihilated and in that moment the hivemind was broken as well leaving the tendrils and offshoots now to fend for themselves.  

This did not come without cost though,across the Imperium warp storms erupted with the largest about the Sol system cutting it off from the rest of the Imperium and potentially killing everyone in the system.  Psykers across known space also reacted violently to sudden visions which wracked their minds.of titanic forces within the Immaterium clashing with their greatest heroes about them.  The Emperor has entered the warp now and does battle with the gods of chaos, tying up their attentions and forcing them to draw their greatest champions to them just as the Emperor has.

This now leaves the Imperium vulnerable for without Terra and it's centralizing authority, with the Artonomicon, without the God Emperor of Mankind or the Chaos Gods.  What force is there to unify humanity and stop every petty lord from taking what he can?