Setup: a few printed maps at different scales, markers, blank paper GM: Present the group with a few maps, ask them to choose two they like. Anything from the galaxy to the local mall. Write the following on a piece of paper with room under each: Premise: Basic idea of the campaign Cast: PC names and descriptions Histories: Past events that affect the present world Mysteries: Rumors, lies, unknowns Places: Places your players know about Treasures: Things the players want to have Contacts: Friendly people the players know Adversaries: Who will stop the players? Ask leading questions until you have a few of each. Offer guidance on what you think is fun, but let the other players do the work. Later, combine these to plan adventures. PCs: pick two maps. The larger scale map is your world, the smaller scale map is your turf. Tell your friends what you would like to see in your adventure, until you have a premise. Ignore the intended setting for your chosen rules. Carve up the maps into various factions. Draw points of interest. Answer the GMs leading questions, and draw those on the map too. Finally, make characters to fill out the cast.
I’ve run 4 campaigns based on this and it’s always made for a great time. The map and worksheet make for a great GM resource, and kicks things off with all the players engaged in the world, and wanting to explore it. What started as a way to kick off rule-testing before the theme was settled on my games has become the de-facto way for my group to start any campaign, regardless of the intended rules.