
Here's how I made my last TMNT/Mutant Future adventure: I looked at the character attributes in the game (strength,intelligence, etc.) and flipped randomly through the skill list and tried to think of a situation where each attribute and/or skill would be useful. About 15 minutes of thinking got me about a session's worth of material.
You could probably brainstorm a whole megalocale or campaign pretty quick if you just tried to get at least one situation for everything on a game's skill list. Call of Cthulhu seems particularly ripe for this treatment.
Not that the PCs would have to do all that stuff to get through the game--it just seems to spur the imagination to try to think of how a gameworld where both artist: stage magic and pilot: hovercraft could both be useful in short order.