For the setting of this game, I would select a specific region and leverage the available maps to really flesh out the damage done during the apocalyptic event. I would keep the area limited so that I could breath real life into it, instead of making the entire state (or country) a sandbox.CRITTERS:
I think I would develop an effect or exposure that sets your monsters apart from the standard PFRPG. Like a radiation or a toxicity template. Also, I'd create some weird or alien effect that boosts the intelligence of critters that normally are not.
You might want to consider allowing Psionics
. It certainly fits with the feel of the game. I like what the Dreamscarred folks have done with Psionic rules for PF.