This is a pretty great list of RPG Plots, and some good thoughts / advice at the bottom, too.
What follows is a scrap of trivia . . . my collection of RPG plots, in abstract form. I built this by examining the premises of hundreds of published adventures for all systems (including those systems dear and departed from print), trying to boil them down to common denominators. The results are presented here: arbitrary, and sometimes redundant. Nevertheless, I turn to this list when I'm stuck for a fresh premise for next week's session of my campaign, whatever that campaign might happen to be about at the time. It helps me keep from falling into thematic ruts (my least favorite kind). With any luck, it might serve a similar function for you.Big List of RPG Plots
Worth a gander. :)