Here's the
BBC announcement that Tennant's leaving after next year's specials. I wish I could say I'm shocked, but I'd already guessed it from what was said at the end of season 4. I still think it would have been better for him to leave straight after season 4 instead of making the show do this weird quasi-hiatus thing for a year for him.
I have the stupidest wish that Eccleston would come back.
Sigh.
[Okay, maybe not stupid. Romana II was regenerated to match one of the Doctor's companions.]
But bringing John Simms back as the Doctor? Would only really play out well if Martha were still the companion and they had to work past a lot of her looking at him and flinching because that face used to be the Master's.
I wish it could have been Catherine Tate - they had the set up for it.