Although he'd have to jump forwards a few years to do that, I think, and drag the entire Discworld scenery along - the settings are more tied together than in the beginning of the series, when he could easily make all Discworld except for one tiny country make a 15 years jump in one book so the young hero starting out as a baby could return as a young man only few years (if at all) later. I don't think I want a Discworld version of the Disney Babys, and not many other grown up readers would. Especially in the Watch environment, which always had some whodunnit atmosphere.

As for Granny - he hasn't come up with any new ideas for this character for some time now. As I said, a worse writer wouldn't need too many new ideas and just make it do the same stunt again and again in a different frilly dress... Granny has always had more of a common theme underlying her stories, but in a way that worked, and the stories themselves never had a "second warming up" feeling. Still, now getting a story with her in it that makes me go "Whoa, how totally unexpected" would surprise me.
Everything's possible.