A Sandbox RPG taking place entirely in Ahnk-Morpork...and of course Death's Cottage (assuming you meet the requirements to get there)
Ahnk-Morpork is an epic fictional city with a LOT going on and a LOT of options for an RPG. Taking the Dragon Age: Origins route, you could choose to start as a tween Wizard starting out in the U.U. Or as a young woman who just come of age and is starting a career with the Seamstress Guild! Many would likely join the Assassin's Guild of course...or Thieves...or The Watch...or the AM Postal Service...I know people have been clamoring for a Discworld MMO and I think that limiting it to A.M. would be a great starting point. Of course the game I envision is single-player >.> and allows you to enter every building via all of it's doors, windows, sewer-drains, etc...