My basic assumption was that most, if not all, of the characters are going to be paranormal. This is immediately a restriction on character generation: all the paranormals have certain personality traits in common: they are highly intelligent, competant, and self-willed. They are generally curious by nature - poking into any sort of secrets and activities they can find.
In addition, the PC's needed to be the sort of individuals who would stick together, for practical reasons. While my general idea was that the world would run itself, I decided to choose a starting situation to push the PC's together and get them introduced. This was certainly a dramatic nod. I decided by fiat that they would all be contacted by someone who wanted their help.
My starting situation for the campaign was that the PCs were each paranormals in the Chicago area who were contacted by a man ( Kurt Fenlon) who had escaped from a world-spanning secret society. He wanted their help in finding out what the society was up to. Now, as it turned out, in play they ignored the society after three or four session. The shared knowledge was enough to pull them together, but they never did investigate the Society. Instead, they took off in a completely different direction.
We were using the Hero System rules, 4th edition. As a bit of background, all of the players and I had played in several Champions campaigns together by that point. In comparison to these, I had a fair idea of what I wanted and what not. Characters were designed largely by individual discussion between the players and me. Since only I knew the secret of how powers worked, I was the one who defined the specifics of the powers as well as how they were written up in Hero System terms. I asked each players roughly what he wanted, and I then gave him the closest approximation I could think of to that. To make up for this, I tried to be generous in what I gave to them.
The first character, I think, was Craig Neumeier's character Nomad. He told me that he wanted something like an immortal from Highlander, who had lived through centuries. This was a problem for me because in my secret explanation, powers only began around 1898 at the earliest. However, I then realized that there was a small window which allowed for these. I knew that the Keystone had been put in this universe before 1898, but I hadn't established details. I wrote in that the event was in 634, and that the Keystone had a brief moment of action before being shut down. As I refined my background, this became an important element. My idea was that the Keystone was programmed with a moral obligation not to modify human life in general, but that there was logically a narrow time during conception when it could. There were a dozen or so individuals who were being conceived at the moment when it was imprisoned. These people grew up to find that they were immortal, as well as cursed with a wanderlust of sorts. In fact, their abilities were a makeshift attempt to help them to find and restore the Keystone. These immortals were distinct from paranormals in that their abilities were built into their shadow or "soul" as an independent virtual machine.
I wrote up the immortality package in Champions terms, and explained to Craig the wanderlust and sixth sense that came with it, without of course explaining why. I think Craig handled normal characteristics and skills for his PC. This actually fit reasonably well with the mysterious purpose of immortals in Highlander.
The next PC was Chris Lehrich's character Shroud. Chris had the idea of a martial artist with a ghostly body who would surround himself with darkness. Now, this was a mystical idea which didn't directly fit with my pseudo-scientific explanation.
It was not a perfectly realistic campaign. There was a bunch of hand-waving explanation of how superpowers worked (a bit like FTL in a sci-fi game). Aside from the powers and the whole backstory to them, I had two main differences from the real world:
The following are sample bits of background similar to anecdotes I gave to my players.