I was listening to the Unspeakable podcast, a podcast about Call of Cthulhu, and noted Dennis Detwiller had some guides for scenario design.
He suggested
1. having a situation
2. a plan for what would happen if nobody else got involved
3. outlines for all the different ways you could investigate and interact with the situation and sketch possible results from that.
It sounds like pretty good advice. Maybe it's all old news to you, but it made me think how it could apply it to some different kind of games and not only Call of Cthulhu.