I will post messages on specific portions of what I am looking for to start delving deeper into this campaign idea.

First lets start off with some basics. What do I want to have in the game and what do I want the players to be able to access and use. I already have pretty much decided on Pathfinder as the RPG of choice. Using everything that it has to offer. In addition to the core books it lets me draw from D20 Modern and Dungeon & Dragons 3.5 for rules and Ideas as it is very much backwards compatible. The ideal would be to let players chose any of the available classes so we'll need reasons that Arcane and Divine magic work. I will, for ease of use, not have any psionic abilities in the game... most of them can be duplicated with spell and feat anyway.
Races allowed gets a little trickier. The game includes several different races available for play but if we set the game in modern day earth well then we have to be a little crafty to allow anyone to play anything other than a human. We can open up the areas with a few small loopholes in the world such as the belief and tradition of Faerie as another plane. The events that shape and caused the game to be where it is currently could also do something like weaken the veil between the worlds. This would give a basis for allowing Magic to return to the world as well as give access to things of myth made flesh.
To give the players access to all this it may be best to include the mass destruction of the world to the intro of the game and not the game itself. The game would be more about how the players survived against great odds and the rise of new evils to plague what is left. Because remember the basic Aztec belief meant not the destruction of the world but the destruction of what the world was into something new.