Shared worlds, from what I understand, are a colossal pain in the ass.
There's a series of books out there. Thieves' World, edited by Robert Asprin and Lynn Abbey. They're pretty good. They're essentially series of linked short stories about the people in the fantasy city of Sanctuary -- crooks, bravos, bards, whores, nobility, the Prince who rules the city, One-Thumb, the bartender of the Vulgar Unicorn bar, and many others. They're hard to find, though, these days, because they're out of print, and it seems unlikely they'll go back in anytime soon in their current format.
This is because Thieves' World was a shared universe. Essentially, Asprin and Abbey drew up and created the city of Sanctuary, a backwater of the corrupt Rankan Empire, and invited all these authors to create characters to fit the city, and go nuts. Naturally, the authors began by creating their own characters... and having them interact with OTHER authors' characters... and the stories began to write themselves. They were quite popular, back in the day.
They're out of print, now. Robert Asprin's dead, and the thing apparently depended on his willingness to keep in touch with all the authors, juggle copyrights, and keep the publishers and authors and their lawyers happy. They were good stories, but apparently agents and publishers in particular insist on the legalities... which are apparently a major pain to manage. If you want to read them, you're likely to have to haunt half-price paperback houses.
This became evident to me when I tried to buy a copy of the Chaosium Thieves' World boxed RPG set, back in '83 or '84 or so. It was already gone. They'd gotten the rights from Asprin and his publisher and all the authors... but made the mistake of saying "How could we make this product as massively useful as possible for as many gamers as possible?"
Simple. They included stats for every roleplaying game they could think of at the time. You can find stats for Hanse Shadowspawn, Lythande the Starbrowed, and everyone else for Dungeons and Dragons, Traveller, Runequest, Tunnels and Trolls, Advanced Dungeons and Dragons, and more. They included Poul Anderson's excellent essay on fantasy literature, On Thud And Blunder, http://www.sfwa.org/2005/01/on-thud-and-blunder/ and a whole bunch of other excellent stuff, including, of course, an entire prebuilt city for adventuring in. A friend of mine bought it for use in his D&D campaign, and I lusted after it from day one.
But when I got around to going and looking for it, it was gone. Apparently, some of the game publishers whose systems had been mentioned in the supplement got a bit bent about the fact that no one had consulted them before allowing their stats to be used in the thing, nor were they getting any royalties.
The creators of the boxed set said, "But wait. We didn't include any of your rules. We simply created characters, USING your rules, and published them. Your copyrighted material does not appear in our work. Furthermore, in order to USE our work, the gamers will have to go out and BUY your rules!"
You'd think this would make sense, but apparently, someone bitched enough that the Thieves' World RPG supplement ceased publication. Bidding for old copies is spirited on Ebay. Chaosium did not reprint it, nor are they likely to.
Same thing happened a couple decades ago when Wizards of the Coast published The Primal Order, a system for creating religions and cultures for RPGs and fiction. They made the mistake of thinking, "Wouldn't this be useful if we included information that allowed you to plug this into existing roleplaying games?"
They did not include D&D, of course -- D&D had gone through a litigious phase in the late eighties and early nineties, even suing people on the internet for talking about D&D -- but apparently, other game publishers were just as snarky, and The Primal Order quickly ceased publication.
It brought to mind IndyClix, the HeroClix game that included characters from comics other than the Big Two publishers. No one sued -- WizKids (the HeroClix company) had been careful about securing the rights -- but apparently, it was just such a big pain in the butt juggling all these companies and their lawyers that WizKids just said, "Hell with it. Too much work for too little profit." There would be no further expansions for IndyClix.
Sigh. Why can't we all just get along? Is there no hope for collaborative creativity?