Well in truth we wish – what an exclusive that would have been.
We could publish the list of contenders, but that is what twitter is for! (My personal favourites are Julian Sands, John Rhys Davies and Sue Perkins by the way).
Rumour has it that the Doctor has already been cast, and with a little reading between the lines we suspect they are a key cast member of a very popular TV show who will be announcing his stepping down this week (possibly tomorrow).
This is all rumour and conjecture. We wish we knew. We will find out when the time is right.
Until then, enjoy the ride.