I read something online today, suggesting that Sir Terry's daughter is going to take over the Discworld authorship. I really hope this is true.
She's co-owned the characters & concepts for a while, i believe she set up the company Narrativia with her dad, which deals the licensing rights.
I'm not sure whether that means she will write new books, or continue just managing the "discworld" property. She has been invovled in a couple of TV adaptations over the last few years.