The new Doctor is a guy called Matt Smith, who we don't know much about.
This is usually a good thing. Apart from Casanova we'd never seen Tennant in anything and Davidson had only been in All Creatures Great And Small (that we know of).
This will of course be the 11th Doctor, so only two more unless they work out a way round the regeneration problem.
As we said before, it never stopped the Master. When we first met him, played brilliantly by Roger Delgado, he was on his last incarnation. He got round this by taking over the body of Nyssa's father, and was played by Anthony Ainsley.
In the 8th Doctor movie we see him as some snake creature, after his execution by the Daleks, then taking over the body of Eric Roberts.
Then we see him up against the 10th Doctor, first as an ageing Derek Jacobi, then regenerated into John Simms (but why no goatee, John? Travesty!)
So we know the Master can regenerate past the 13th incarnation.
We wouldn't expect the Doctor to steal bodies, so does that leave him with some legit method to crack the problem?
Like we said before, we don't consider the McGann film to be a real part of the Doctor's history, any more than the Cushing films were. If we never had Ecclestone, and Tennant had been the Millenium Doctor from 2005 (as he should have been) then Smith would be the 9th Doctor.
This also raises the issue of will we ever have a black Doctor or a female Doctor?
Without making a bad pun - time will tell.