I saw Russell T Davies interviewed at the National Theatre 2 weeks ago, and he said that despite the various rumours, no-one has been cast yet and, when they are, he will have nothing to do with it as he's leaving too. All the names that have been mentioned are just pure rumours.
Ages ago, long before Tennant announced his departure, I thought Rhys Ifans would be good. Funnily enough, he's one of the 'rumours' but nothing to do with me, honest. Now I reckon Richard Coyle would be the best candidate.