Discworld RP Interest/Ideas/Discussion

So, I've been thinking of making a Discworld RP. 

Thing is, I'm not sure how we'd do it. Would we control canon characters, make original ones, both? Reasonably certain the setting would be Ankh-Morpork, though.  

One way it could be might be some kind of murder-mystery, Watch style, that involves pretty much everywhere on the Disc, or just Ankh-Morpork, which constitutes pretty much the same thing anyways. Any other ideas are fine, though. 

There are official DW RP Gs out there, if that's any help. Also a MUD.