Last year, I was in an epic multiverse-spanning Marvel Heroic Roleplay campaign run by Doc, which had a flexible 'cast' of characters allowing players to drop in and out as suited them. While that campaign was in progress, I'd had plans to run a one-shot every month, and one of the one-shots I had planned was a Doctor Who: Adventures in Time and Space campaign, which - in a collaboration with Doc - I had planned to tie-in as a crossover with Doc's multiverse campaign. During the adventure I would introduce elements of my own, which Doc could then integrate into his own campaign, and vice versa. (One of the players used his 'extra-dimensional pockets' to procure a sonic screwdriver, which I would have referenced in my one-shot.) But sadly, there wasn't much interest in one-shots at Saturday GUGS at the time, and the plan fell through. Doc did eventually run a few one-shots of his own which referenced his main campaign, and one other player did say they might carry on one of the plot threads from that campaign in a one-shot of their own, but not much else has developed since the campaign wrapped up.
I've had experience of intra-society metaplots before. The University of Stirling's roleplaying society had a couple of 'shared campaigns', one in the Warhammer Fantasy setting, and one in our own homebrew urban fantasy setting. Again, players could drop in and out of either games as they saw fit, they might have two or three different characters to play depending on their mood or where the game was set, and there might even be two games running within the same setting each night. Events which happened in one game might be referenced or built upon by a GM in another, and sometimes GMs would get together and brainstorm events or plots which might affect the world as a whole. It was one of the things I really like about gaming at Stirling, as it gave more of a sense of community between groups and also helped bring those game worlds to life.
It is, of course, difficult to set up such 'shared universe' games in larger societies like GUGS, where there are so many different gaming groups and each one might be playing in entirely different settings. You could circumvent that by using the Multiverse as a plot device, but that can be overused and get ridiculous very easily.
Still, I feel like there's some potential in the idea of GMs from different groups comparing notes and integrating elements from each others games into their own campaigns if they happen to co-exist in the same setting, like Legend of the Five Rings and World of Darkness. Just needs a bit of communication and organisation. I may experiment further with the idea in future.
I'd be interested to hear other peoples' own thoughts on - or experiences with - shared universe roleplaying, so please leave comments below if you have something to add. :)