Another RPG one-shot I'd like to try is a historical swashbuckler. Too many RPG sessions turn into exercises in accounting --- players worry about their limited resources, play it safe, and end up forming the traditional semicircle around the bad guy and keep hitting him until he's dead. I prefer characters who take risks and show some panache.
First question would be the time period. Lately, I'm back to reading the Flashman books by George MacDonald Fraisier, ripping yarns about a Victorian British Army officer and his adventures defending Queen and Country, so the 19th century is an option. Caribbean pirates and 17th century musketeers have the advantage of familiarity. Were I a more ambitious man, I would maybe try something during the late 1700s and the Age of Revolution --- the PCs are secret agents working for Jefferson's Illuminati. Something with Irish and Scots rebels during the Jacobite Rebellion would make for great accents around the game table and a terrific soundtrack, but I don't know much about the period.
The new Star Wars Saga Edition would actually provide an excellent starting point, rules-wise. A company called AEG did a d20 book called "Swashbuckling Adventures" which had a lot of problems (a poor grasp of the d20 mechanics, and a setting that threw Vikings, Robin Hood, musketeers, Zorro, and Napoleon together), but it certainly had some material worth mining.