The obvious answer, of course, is that we've already been introduced to the next Doctor and that it's John Hurt. Which would be pretty damn spectacular, frankly, and would also imply some sort of tangling of the timeline given that the eleventh Doctor was already clearly aware of who - or what - exactly Hurt was. It would also imply a much darker turn for the character so let's assume that the BBC isn't going to want to mess with their biggest cash cow to quite that extent - which seems a safe bet - and that Hurt, instead, is going to prove that catalyst that leads to the next regeneration and / or the villain of the upcoming season.
If that proves to be the case, then one of the most iconic roles of all time is in need of a new actor. So have at it. Who should it be? Do we go comic? Do we go darker? Established? A new face? Is now the time to insert some racial diversity into the role? Who do you want to see? Who do you think is likely? These two things are quite likely not the same, so let's lay it all out there!