Had a vague idea, this is it barfed out.
As a group (GM with players):
1. Describe three or four elements of the setting - political factions, authorities, subgroups, professions, cultures, locations, an aspect of the natural world, 'magic', geography.
No more than 10 words each.
At least two should be groups of people; all should interest the folks at the table, and each should have a significant relationship with at least one another.
e.g. Dwarven Miners who inhabit Limevault Pass despite the Relentless, Bone-Searing Winds and The Men of Cobblevale in the foothills, below.
2. Describe one or two major forces that will overturn the balance of power between these elements.
No more than 25 words each.
e.g. Dwarven numbers have declined to the point where they can no longer maintain the appearance that Limevault is well defended.
e.g. The civil war among men is over, and Cobblevale has overflown with adventurers, veterans, mercenaries and ne'erdo-wells all looking for quick coin.
3a. Name a group of protagonists and the perfect opportunity they now have to do something significant (central goal).
3b. Name an antagonist who will do everything to prevent this, to preserve the status quo, or to bring about some opposite vision.
4a. Name an antagonist and the terrible villainy (however overt or subtle) that they can now commit.
4b. Name a group of protagonists and what they will do to thwart the villain (central goal).
e.g. Gzeyrik the Mad and his retinue of Knotbeards have decided to descend from the pass and teach Cobblevale the lesson in humility it deserves.
e.g. Sturn of Melk, freshly returned from the war, cannot believe how Cobblevale's defenses have atrophied, and will do whatever he can to shore them up to prevent the chaos of the west touching his homeland.
5. Decide which of these two groups will be the party!
Together, but individually:
6. Make a character defined by the central goal.