I don't think he's derivative, I think he's self-consciously referential and indeed intertextual, acksherly, Twiglett.
In fact I was talking about JK Rowling the other day to a friend and saying how Pratchett's very intelligence and ability to play about with quite major stuff - both in terms of other literature and in terms of Bigger Stuff (if of course you get Bigger Stuff than books) - makes his popular success quite surprising. Other novelists who are similarly intelligent about fantasy haven't managed it, I think.