I've been meaning to develop a supers setting for GURPS for a while since I wanted to see how well the system works with the high-powered supers genre. The goal was to allow pretty much every common power source / origin defined by GURPS Powers, GURPS Supers, and superhero comics in general, yet have a strong, interesting "theme" that ties the world together.
And, after recently re-reading the comic series Planetary, I think I have come up with an idea.
This world exists somewhere within the Infinite Worlds cosmology of GURPS, and is, in fact, a close parallel of Homeline - even sharing the exact same "now", which means that parachronic travel (moving between different timelines) from this world is possible. All that was needed was the inventor... and the man who invented parachronic travel in Homeline, Paul Van Zandt, also did it on this world, right on schedule in 1995.
Also like in Homeline, he concealed his experiments from the public. However, while his Homeline equivalent published his findings in 1998, on this world he refused to do so - instead, he stole technologies from other worlds and released them as "inventions" via his company, White Star Enterprises, and quickly became the richest man on the planet.
However, he only released a fraction of what he and his fellow conspirators have acquired, and gradually faded from public view. The wealth was only a means to an end - what he was really after was human enhancement technologies. He, and his Inner Circle of trusted associates, wanted to become like gods. Quite a few parallel worlds had experimented with different forms of human enhancement, and combining them held enormous promise - but all these approaches needed thorough testing, and so the Inner Circle turned their own Earth into a gigantic lab. Mutagens, nanotech, even alien artifacts were all raided from other worlds and turned loose via shell organizations or staged "accidents" that exposed normal people to them and turned them into super-powered beings. Meanwhile, they raided top scientists from other worlds and forced them to work in their secret labs on the latest scientific and technological breakthroughs.
It was only a matter of time until one of Van Zandt's raiding parties came across one of the other crosstime traveling factions, and luckily he came across the Cabal first - who were very understanding about this whole "experimenting on humans in order to become gods" thing. While the Cabal believes that eventually all this will blow up in Van Zandt's face (which is why they haven't formally invited him into their club), they have become eager trading partners for knowledge and information. They warned Van Zandt about the other factions in the multiverse (causing the raiders to become a bit more circumspect), and helped him with magical advances - even finding out a way of raising his world's mana level.
As a result, around the turn millennium super-powered beings have started to appear. Their origin stories differ widely, and often seem mutually contradictory (since the artifacts triggering them come from different timelines). The Inner Circle generally allows them to operate freely so that their powers and their interactions can be observed, only intervening in particularly interesting situations. And the supers - villains and heroes alike - go about their daily business without realizing the hidden puppet strings that control their world...