Having a long running persistent world, here are some those elements I've found indispensable. Today we'll talk about the timeline and the world it creates.
1) A general timeline.
This assumes you you're aiming for a near real world setting. I've made a number of changes to the game world's time line. I didn't write up a bunch of specifics, rather a bunch of places where I can make changes. Prior to World War one (ww1) science/tech characters and those who's powers ate derived from science generated strangeness are limited, so magic is dominate power source. Magic was stronger the farther back from the modern age you go. I break history into four groupings. All of these eras have their own proper names, however a bunch of these are disputed on the exacts, and I don't care to devolve into that sort of discussion (for example I'm western centric AF). The ancient period, everything before the bronze age collapse. Magic is everywhere, and does all kinds of crazy things. The old historical period, bronze collapse to rise of Rome. Magic is fading, but still dominate.The historical period, Rome through to the about 1450 AD (roughly the beginningof the renaissance, everone argues about when that begins,so I put the flag there). Magic is basically what you find in the HU book or magic items crafted by using ley lines and nexus points. 1451 to the near future is the modern period. One could also toss in the far future period, but that's never been my thing.
While I still make use of mutants and experiments, they draw their powers from magic, and blood lines instead of a mutant exposed to radiation or what have you, they are the descendants of demi-gods. Alexander the Great claimed to be related to Hercules, so a PC mutant could be too (or any thing of that sort). An experiment would be alchemical in nature. Aliens are dimensional on nature, rather then space. The closer to the modern age you get, the more like the core rules things become.
In the ancient period, make up whatever history you want, no one can tell you you're wrong.
Real world stories (such as the epic of Gilgamesh, a bunch of Egyptianstirues, Jewish/old testament christian stories we're familiar with and good grief China and India's stories are absolutely off the hook) are end to end crazy powerful magic and characters who's abilities make optimized PU2 immortals look weak sauce by comparison. The old historical period, you have more structure to work around but still yards and yards of room to do your own thing. More demi-gods and superhuman/supernatural beings, fewer straight up gods in this era. With the rise of Rome, real world stories are still full of people who are more then human, but roman heroes are just less then the characters of the Greek era before them. The world if this period greatly informs or outright dominates the modern world. Lots if documentation means you have less room to work with and stories start to take on a war in the shadows kind of deal.With the advent of Christianity superhumans are either channeling the power of the Christian God or the foul demonic magics from the devil. Creatures we'd call lycanthropes (though the story of the werewolf comes from France of 1550 AD) vampires (which predates the book Dracula) ghosts and a great many of the monsters of ... um, let's say Palladium Fantasy RPG all spawn from this era. Additionally, you have a number of literary characters that were written about in the modern era, were written to have existed in this era. Magic fades into the shadows, and publicly is the domain of the deranged and alchemist hiucksters. Finally we have the modern period and characters would fit with the most any comic book interpretation.This covers our origin of characters. More later.