I've always wanted to run a historical mystery/detective role play. Something akin to Name of the Rose, a locked room mystery set in a single local, where the players have to figure out who killed a particular NPC. Or alternatively something where everyone plays as a medieval peasant trying to find out who's responsible for acts of witchcraft taking place in their village.
The first one seems interesting enough although detective RP sounds more like something of a duo rather than a group now the second one I smell gasoline on the air on that because i know how people will generally react but who knows maybe it doesn't go so south...