To expand upon my post in the other thread (a combination of Rifts and Warhammer 40K)
The Apocalypse wasn't from a war, but from trying to prevent it, the point of which is that sometimes events are inevitable. In short, arcanists trying to prevent nuclear annihilation called upon forces beyond their control from an altered book of spells. Mutations are caused by raw Chaos, from either exposure or worship of the deities of Chaos (I plan on using the original five from Warhammer, Malal being the fifth). Dimensional portals are rare and guarded now, but during the Infestation of Chaos, were open, and many strange creatures wandered onto the planet, some sentient, some not. Pure Strain Humans are isolationists and rarely allow spellcasters or non-humans into their domains, but they are not as mindless about this as the Coalition of Rifts. Hordes of Chaos worshipping creatures roam the wastelands and deserted towns and cities, preying on the weak and salvaging any old technologies they can. Those dedicated to fighting Chaos band together and hunt down these hideous monsters. And the players will have the freedom to choose.
I am tossing around the idea of starting the game off early using just d20 Modern, and taking the game pre Apocalypse up to the main event. Nothing too campy, nothing goofy, all serious and gritty.
hellbender