Civilization/WMG

Civilization V is an experiment of some very technologically advanced civilization.
It all appears to be a kind of a social experiment. Famous leaders are resurrected and given immortality to lead the civilizations, and ancient ruins are found everywhere, perhaps remains of other, less fortunate experiments? Additionally, Nebuchadnezzar II seems to largely confirm this theory with what he is saying. He may occasionally say something among the lines of "These fools outside seem to think I am a god of some kind. It's unlikely." and, if you defeat him, he may say "Good work. I shall put in a good word to the demons for you" or "It is over. Perhaps now I shall have peace, at last." as if it's not the first time he's going through this. He may be indicate some outside observers with his sayings about the demons. He seems to be the only sane guy in the game that may know what is actually going on.
 * Nebuchadnezzar probably isn't an example. His erratic behavior is a reference to the Biblical portrayal where he goes insane and lives in the wild for seven years.
 * However, this does veer into Fridge Brilliance when you consider the game over text of Civilization 1.

Civilization V takes place After the End
Think about it. The world is, from the start, dotted with ruins from ancient civilizations, which your units may scavenge for gold, works of art, maps of their surroundings, survivors or, more importantly, equipment. At the start, it isn't anything suspicious, just regular artifacts like bronze spears and bows. The horrifying consequences however come as you climb up the tech tree, and said "ancient ruins" now possess muskets, cannons, and, worse of all, tanks and assault helicopters. This clearly indicates that this "ancient civilization" whose ruins belong to possessed a high scientific and industrial level of development, only to suddenly disappear without notice. My theory? All these ruins belonged to a single extremely powerful and advanced civilization which spanned the whole globe, which would explain why all of them possess the same architectural style and why they're found in such unlikely places as islands isolated in the middle of the ocean, or in the polar caps. At some point before the start of the game, however, something catastrophic happened to it, outright destroying most of its infrastructure and killing most of its population, wiping its cities off the face of the globe and leaving nothing but ruins dotted around the continents and a few pockets of civilization, which while retaining enough population and infrastructure to survive, turned into xenophobic city-states highly distrustful of each other, and bands of survivors driven mad by the shattering of the very world they inhabited, which turned into the barbarians. The player and the A Is, on the other hand, are survivors which managed to gather enough materials to create their OWN cities, and start anew. The Fridge Horror, however, comes from the dawning realization: What catastrophe could be powerful enough to wipe out an unified, world-spanning culture with highly advanced technology, leaving nothing behind but little pockets of civilization, while leaving the environment intact? And worse... What is keeping it from happening again?
 * The Reapers did it.
 * Alternatively, the world you play in is just the result after the previous great civilization gained a Science Victory.
 * Perhaps the civilizations starting out are the survivors from Alpha Centauri returning to recolonize Earth?
 * Also worrisomely, the computer has a tendency to spawn ruins in close proximity to the Barringer Crater.

The leaders in-game are a front for their nations' personifications.
Given how all the games take place over thousands of years, it's highly unlikely that (even with video game time) the historical leaders available to you are actually immortal...that is unless they're not human. On top of that, you're given absolute control over just about everything in your civilization, and effectively serve as the only constant through the ages. And the only way you could "die" is if your nation is either conquered or nuked to oblivion. Of course, all that stuff is behind the scenes to cover up the whole thing. The leaders, really, are just for show in the diplomatic screens.
 * Those That Came Before from Assassin's Creed II?
 * Perhaps the Nations are connected to them?
 * Maybe in a Hetalia~esque manner?