Doctor Who: "Silence Will Fall"

Following The Pandorica Opens there has been much speculation about ( a top ten list of spoilers. )

Now, it's quite possible that the voice belongs to some person or entity we've never previously encountered. (Or to Prisoner Zero, or Sutehk, or Fenric. Or Soldeed.) However, I will be more surprised if it turns out to be somebody no-one has thought of than by it being anyone on this list above.