3) Monsters originated out of the Pits. The Pits are the great craters left over from the bombs; they are centers of chaotic magic, and the areas around them are very dangerous, due to the strange magic and the monsters. Essentially, many ruined Ancient cities are vast dungeons in the strange "Underworld Dungeon" way. Many monsters have spread far and wide and settled in far from the Pits in the centuries since the Change.
4) The Old Gods returned (taken in form from the classic 1E AD&D Deities & Demigods book and, for the demi-humans, the expanded lists in Unearthed Arcana). Europe, Africa, and Asia all pretty easily settled into their Old Ways, with no little trouble along the borders. In the New World, however, things were quite chaotic, especially in the USA. There is a generic "Temple of Angels and Saints," which is led by a council of Seven Archangels; similarly, the classic Lords of the Nine Hells are also present. No sign of the Big Guy or Satan, however... nor of Eru, the Valar, and the Maiar.
5) There was a Dark Age of about 200 to 300 years; in some places the Dark Age still rules. In others, great empires rose and, in the time since, have fallen. The general divisions of cultures and kingdoms in the former USA is loosely based on this map:
The regional map for the campaign is based on this map:
The Empty Coast is home to monsters and centuries of lost treasures of the fallen Celestial Empire, as well as to the Dark Pit of the Black Pagoda and the Hell Pit of Sanjo.
The Republic of Sanrosa and the Republic of Reyez are elven realms; Sanrosa is closed to other races, while Reyez is open to trade.
The Khanate of Montray is ruled by the Hells Angels. This nomad nation is descended from the military arm of the Celestial Empire; the city of Lost Angeles was the fief of the Angel Khan and became the center of his kingdom when the empire fell. 50 years ago there was a coup, and the Khan and his temple the Temple of Lei Kung, were overthrown by a distant cousin and his sponsoring temple, the Temple of the Nine Hells. Thus began a decades-long war against the neighboring domains, finally culminating in the invasion of Stockton and the Battle of Beerbridge, where the Great Khan and his core followers were slain. His empire collapsed, and today the Khanate of Montray is one of more than a dozen petty domains that have sprung up from the ruins since. As the Celestial Empire used orc mamelukes in its armies, the ruling and warrior classes of the Khanates are a thorough mix of humans, half-orcs, orcs, and goblins.
The coastal mountains being home to many flocks of griffons, the valley is home to no few knights and nomads mounted on griffons, hippogriffs, and demigryphs. The Sierra Mountains are home to dragons, mostly red and white, while the forested foothills are home to green dragons. Blue dragons sometimes make forays over the mountains from the Nevada Wastes, while the Great Fen, Sanjo Swamps, and the Salt Death are home to black dragons. The savage Aztlan warriors of the eastern slopes of the Sierras ride dragonnels, while their barbarian Californio neighbors of the western valleys ride giant eagles.
A generation after the disaster at Beerbridge, the Kingdom of Stockton and the Khanate of Montray are both girding their weapons and readying for war. There are strange things pouring out of the Demon Pit of Sacrament, and unusual activity in the Empty Coast. The elven seers of Sanrosa have exchanged secret missives with their counterparts in Mendenhal. Strange ships have been sighted on the coast. And the last three seasons have been very poor for the mountain folk... trouble is a-brewing...