Doctor Who What If David Tennant Is No Longer The Doctor?

As many of you will know, at the end of The Stolen Earth, The Doctor (David Tennant) starts to regenerate, as I'm sure fans will agree, The Doctor was only hit by a glancing blow from a Dalek, so why is he regenerating? Lets explore this further...

What It...

The Doctor is regenerating, and we are to have a new Doctor?

We know the end of The Stolen Earth is a very well kept secret by the BBC, we know David Tennant is still in Doctor Who, from the photographs of filming the Christmas special don't we?

But what if David Tennant isn't playing the Doctor, lets say he is to be a bad guy instead, the Master perhaps? There are rumours that John Simm has done some secret filming for the BBC last November, could this filming be to do some regeneration scene?

Well its a theory :)