I do not know who. I’d like to think it’s some unknown talent’s opportunity to shine, because I’m romantic like that. But I disagree with you that they should continue the book cycle themes.
If I were to do it — now I’m not claiming to be anywhere near good enough to actually do it, but for the sake of argument — I’d start off with Young Sam Vimes graduating from the assassins’ guild and Lord Vetinari’s death. (And I would give Vetinari a perfectly natural, boring, old-age death — which nobody believes) and take things their own way from there.
No one should try to be Pratchett. I doubt anyone able to do what I’d like to see, would lack the respect for the man and his genius to try to finish his stories.
That doesn’t mean they can’t create their own stories, with their own characters, and keep the old favorites as a supporting cast.