askold, yeah, from a realistic point of view, the bigger names are highly unlikely, except as a one-off movie special, perhaps. I could totally see Laurie or Rickman doing something like that. As for the continuing series, the most realistically likely of the nominations is probably Mackenzie Crook. He has all the things they look for in a Doctor: a young character actor with an established TV presence on a wildly popular show with cross-Atlantic appeal ("The Office") and a few, but not too many, movie credits.
But can't you just imagine John Cleese as the Doctor?