Because I am very much enjoying running my dwarf game, I decided to start up a second one. I knew a pair of friends interested in roleplaying in a D&D game and were willing to try the same collaborative set-up that the others had gone through. Because much of the creativity in handling a world is shared amongst all the people in the game as opposed to the GM shouldering most (if not all) of it, I didn't feel like I was biting off more than I could chew. Also, I was curious how similar or different the result would be.
Both of my new players came to the table with a general concept they were interested in. One liked the Dark Sun campaign setting, particularly it's grim survivalist bits, its struggle to survival themes. The other put forth a dense dark jungle that was almost too alive, where the trees and every living thing were after you. It quickly became apparent that these two ideas were easily mixed together... taking the tribal us-versus-an-unforgiving-world feeling of Dark Sun and placing that in a dark-continent savage-jungle world. Very quickly, we had a fantastic world inspired by Doyle's Lost World, Burroughs' Tarzan, Haggard's Quartermain, and Conrad's Heart Of Darkness.
Going through the process involved asking the players a lot of questions. What sort of game do you want to play? What sort of themes do you want to touch upon? What sort of setting really interests you? Which races and classes would you want to play? How do those races and classes fit into our setting and theme? Though this might seem like I was tossing a lot of the burden on the two players, I tried to make sure I was asking questions I wanted answered as well; I wouldn't have a character sheet, but I was going to be playing in this game as much as they were, and it had to be a game that captured my imagination as much as theirs.
Once we had our core concept, things started to come together very quickly. Our player chose Shifters as the race for their characters, giving them a noble tribal bent. Family groups forming tribal communities that built small walled villages to protect themselves from the deadly dangers of the jungle they all lived in. Tribes hunted and gathered for food, traded with other shifter communities for supplies, traded off their offspring for prestige and variety, and ruled by their elders. They are a half-step between still beastial savages like cannibal lizardmen (nomadic creatures that prey on the weak and separated) and overly-ritualized civilizations like the god-states (great mayan like cities of fanatical faiths beating back the jungle with stone and fire).
Some of the points, I wanted to purposely leave undefined. They needed to be there as generalities only, as the main focus would be the local tribal settlements the characters would inhabit. Stuff like the lizardmen and the god-states, they didn't need details yet. That could wait until they were actually encountered. But by their existence, it helped better round out the things that would be immediately important. The idea of the god-states, for instance, brought with it the idea of demanding terrible celestial gods, which brought out the idea of earthy savage primal gods, which helped define why the jungle seems overly alive, that it was a creature in and of itself. The god-states and their inhabitants and the gods they worshipped were not important enough to have names yet, but by existing, they gave us explanations for the immediate lands and dangers our characters would care about.
We broke at about the time where we would need to start getting into more specific character details, like classes and powers and stats. The best part of collaborating a world like this is that I didn't really have to put any restrictions on character generation because all three of us had a good idea what did and didn't "make sense" in our new world. I knew, for instance, that neither would consider paladins or clerics appropriate, as those are divine classes unsuited to their primal characters. It was a good sign that we were all on the same page!
By the time we called it a night, I was jazzed for our new game. It had a very 1930s pulp adventure story feel to it. The sort of thing you expect to encounter jungle beasts, savage hunters, massive dinosaurs, haunted ruins, skull-shaped caves, and golden idols. Lots of darkness, lots of thick trees, canopy and roots everywhere, and vines and moss. Our protagonists are feral, Tarzan inspired cat-men, ready to find their path and become men in the traditions of their tribe.
A lot of totally-outdated ideas, a little un-PC in places, and a constant danger that it might slip into laughable camp if not played straight... but rife with potential! And guaranteed fun! :D