I don't normally buy into this sort of thing, but...
Bear in mind that because I try to keep free of this kind of thing I have no idea if you've all been discussing this for months anyway. But I just heard a bit of speculation that's worth mentioning just because it's so ingenious. Please let me know if you've heard it before.
It concernes the identity of Mr Saxon. Or, more specifically, Mister Saxon, since that would be an anagram of 'Master No. Six'. After Delgado, Pratt, Beevers, Ainley and Roberts, John Simm (listed as playing Saxon on OG) would become the sixth actor to play the Master.
NOTE: Please keep comments free of any GENUINE spoilers. Also, anyone who asks "what about Derek Jacobi?" will be hunted down for sport.