PotBS - Dev Journal @ Official Site
Flying Labs' Bert has whipped up a dev diary on creating missions:
How do we make our content? Our content department has numerous approaches to how the various stories get made and end up in the game. Some of our key story lines come from our writers. Others the individual designers come up with and they take them from concept to completion by bouncing ideas off each other and off other departments.
My personal favorites are the ones we do ourselves. Finding the kernel of a good story arc in your head, rolling it around to figure out if it would be fun and if it would fit with the overall theme well, then sitting down and writing it out is both challenging and highly enjoyable. But the coming up with the start of the content is just part of the fun. The real fun starts with the brainstorming session.