Lady Ramkin is another one of Pratchett's great characters. But I suspect she's modeled somewhat after Pratchett's own wife. So he's wisely vague about specifics. She is, however, grossly unmustachian in Guards! Guards! Where she meets Vimes. Her expensive hobby is raising, breeding, and rescuing a type of toy dragon biologically indisposed to basic survival. Or rather, predisposed to exploding due to the complicated biological/chemical process that allows them to breathe fire. That said, I wish he had time to do a book just on her. (She did reform after marying Vimes.)
I'd also like to see him do a book on the Patrician, the Machiavellian potentate who rules Ank-Moorpork. He's so Machiavellian he founded some of the leading cabals conspiring his assassination . He's also the means by which Pratchett offers an insight that out Sun-Tzus Sun-Tzu: something to the effect that the acme of skill is NOT to defeat an enemy without fighting but to negotiate a victory without fighting in a way that the enemy walks away thinking HE won.
Even so, I'm a big fan of Granny Weatherwax, her protege Tiffany Aching, and for opposing reasons the Unseen University (Pratchett's version of Hogwarts) Arch Chancellor and his ever suffering most competent non tenure holding undergrad, Ponder Stibbons. Ditto Vime's troops, Sergeant Colon and Nobby, who are the only character I know for damn certain are based on real, eternal, personalities.
I've often thought of visiting Briton JUST to see Terry Pratchett - like that Spaniard who walked to Roman Italy just to see Livy.