Granny Weatherwax is my favourite character as well. Despite being a witch, she is such a skeptic.
The last book, 'The Shepard's Crown' is scheduled for release this September.
As it is a Tiffany Aching novel, the witches, including Granny Weatherwax, are going to make an appearance.
It does make me sad thinking this is the last book, and glad it is a witches novel, although I would really love another Sam Vines or Moist von Lipwig.
The amazing thing about Moist von Lipwig is that he made such a late appearance in the Discworld, yet he is such a fabulous character.
To comment on 'Mort', while Pratchett is beginning to get into the swing of things with this novel, all of the Rincewind and wizard novels are my least favourite of the whole series. I think the only two I really enjoyed are 'Equal Rites' which is wizards and the first witches novel, and 'Unseen Academicals'. Other than that, I could leave them.
Mind you, I just watched the TV adaptation of 'The Colour of Magic' by Sky, and it was quite entertaining.