Since we have been told that John Hurt would be playing an incarnation of The Doctor in the 50'th Anniversary Special in November, there has been speculation as to which number he would be.
There is the idea that he is the doctor previous to William Hartnell, meaning that William was not the first incarnation -- just the first one called "The Doctor".
There is the idea that Hurt plays the incarnation who fought the Time War, situating him between Paul McGann and Christopher Eccleston. This still means Matt Smith is the 11'th "Doctor", but as with the first theory he isn't the 11'th incarnation of that Gallifreyan.
There was the idea that Hurt is playing The Valeyard, an incarnation of The Doctor somewhere between the twelfth and so-called "final" regeneration. Earlier this month we learned that this option isn't happening as Big Finish announced Trial of the Valeyard as a new audio play. We know that Big Finish is in constant communication with Cardif to ensure that stories don't overlap, so it's impossible that Hurt is playing the Valeyard in November with Big Finish releasing a Valeyard story in December.
I am also wondering how interesting Hurt playing an as-yet-unknown future incarnation would be for fans. There needs to be some link to what we already know..
I don't buy into the idea that there can only be 13 doctors (12 regenerations) given so much has changed since then. We already saw The Master offered additional regenerations (The 5 Doctors), so we know this was under the control of the Time Lords who no longer exist to enforce that limitation.
One future incarnation theory that would be interesting is an incarnation who steals a body the way that The Master did in Keeper of Traken (He stole Nyssa's father's body, having run out of regenerations). A theft of a body in order to gain additional regenerations would be something that would "solve" the silly 12 re-generations problem and would also be something done NOT in the "name of the Doctor".