I don't think Hugo Weaving would do a good Vimes. I have a picture of Vimes in my head, but I'll be damned if I can think of an actor who could play him. Whoever he is, he's not American (not that Weaving is, I'm just saying
).
Magrat Garlick = Emma Chambers (Alice from Vicar of Dibley). Case closed
PS - Judi Dench is too posh to play Nanny Ogg. No matter how good an actress she is.
PPS - For Alan Rickman to be famous primarily for the Boy Potter, I think that would make it time to revolt and kill everyone in sight.