I have this perennial urge to run a campaign based on Michael Moorcock’s Eternal Champion stories. It would necessarily be a huge undertaking which means that it is not very likely to ever be realised, but this is how I would do it.
Each player gets to play one aspect / incarnation of the EC as well as a companion to each of the other aspects. There will be one unique world for each of the players / Champions and the focus of play will regularly switch between these worlds. During the campaign the various aspects of the EC will cross between worlds in the multiverse and meet each other (as well as the more famous incarnations from Moorcocks books – Elric, Corum, Hawkmoon, Erekosë). Given the large number of pc’s and the need to switch between differents worlds and partys of pc’s it would probably be best to have a fairly small group, say three or four players.
1. The group collaborates in creating a world for each of the players / champions. Just make up the basic concepts by contributing one thing / theme each, the player whose champion originates from the world goes first. These concepts could look like this: “Imperial china, civil war, steampunk, zombies” or “Asteroid belt, living space ships that look like insects or deep sea creatures, an ancient empire, invading hordes of mutant barbarians” or “Hollow earth, dinosaurs, decadent immortals, sorcery is a drug”. Draw a very rough map, then let the GM make up details along the way.
2. Each player creates an aspect of the Eternal Champion. If I’d use D&D, the Champions would likely start at level 6 or there about. These are the primary pc’s.
3. Each player creates a companion pc to each of the other Champions (yes there will be a lot of player characters. However, the companions are secundary and will not need as many details as the champions. In D&D they would start as level 4.
4. Each player names at least five important npc’s who have some kind of relationship to their Champion – relatives, friends, enemies, etc.
5. The GM creates one or several npc’s representing the forces of Law and Chaos for each world.
6. The GM creates one or more powerful artefacts related to the forces of Law, Chaos and Balance.
7. The GM makes relationship maps showing how the npc’s connected to the champion are connected to each other and to Law, Chaos and Balance. Some of these npc’s should have connections to other worlds, either those of the other players or worlds from Moorcock (young kingdoms, tragic millenium).
8. Based on conflicts and alliances in the relationship map the GM sets events in motion around the champions. The players decide how the champions react and what independent protagonistic actions they will initiate.
9. The GM escalates the conflict between Law and Chaos, eventually unleashing an apocalyptic cataclysm where the players must decide how their Champions will act and thereby deciding the fate of the world.
The campaign would basically be open ended and very much dependent on what the players choose to do, though the conflict between Law and Chaos would be central.
Edit: The apocalypse would be a situation that the players could interact with in wich way they ever wanted, not a metaplot that the gm should force upon them.