I was thinking... Vetinari must be getting on. Knowing him, he can't possibly leave something so important as the choosing of his successor to the whim of the Guilds or to chance. Therefore, he must already have his eye on a likely candidate; come to think of it, he is probably training him for the job as we speak. So... who is it?
Carrot is the obvious choice, but anything that obvious must be violating any number of narrative causality laws - the Million-to One-Chance Axiom, for one. Besides, there's no way he would accept the job, except maybe during some fearsome crisis, if the city was in dire need of his leadership, and then only for the duration of the emergency. Sam Vimes is out of the question. Someone like the enterprising Mr. Dibbler might appeal to the merchant mentalities of that great city's luminaries, but no. A grimmer candidate would be King Henry; but that would take us a little too close to the real world... Then, who shall it be?
Moist von Lipwig shows some promise. So does Ponder Stibbons, but I doubt he can be enticed away from his High Energy Magic Building. He might be interested in running the University some day, but not the city. Too much non-magical interference there.
But it wouldn't be much of a story if someone like me could guess the answer to this question, now would it? So, if this book is ever written... let's hope it will be... it will probably be someone new, someone that will make his - or her - appearance in some totally unrelated context and will surprise all of us in the end. Can't wait!