Hex crawl idea:
- Decide, as a group, on basic aesthetic (or agree to allow genre to evolve)
- Generate wilderness map (probably randomly), place starting town
- Each player creates one PC
- Each player creates one dungeon and places it on the map
- Each player creates a hook or rumor for their dungeon
- Players decide which hook to pursue
- Author of that hook is referee next session
If a dungeon is cleared, that player makes a new one, places it somewhere on the map, and adds new rumors to the main rumor list.
Map grows organically as needed.
This could be made more complex in various ways, resulting in a richer setting (each player also responsible for a town? a set of fleshed out random encounters? the encounter table for a given region? a custom class or race?), but I think one PC and one dungeon per player is the fertile core. Rotating referee duties spreads the prep load around and works against overly comprehensive world building. The juxtaposition of different styles creates a world with more diverse influences.