It will be a shame if Matt Smith does leave - I had thought no-one could replace David Tennant (although, to be honest, I found his characterisation getting annoying during the specials between Seasons 30 and 31 - but maybe that was intentional), but Smith has managed to do that.
Doctors come and go.
A Doctor needs to go at the right moment. With hindsight, Tom Baker's tenure went downhill after The Armageddon Factor, and I wonder if that would have been the right moment for him to leave. Fans often question why Romans regenerated between that and Destiny of the Daleks, but I had assumed that it made sense on-screen (off-screen, of course, Mary Tamm had left and Lalla Ward was replacing her) as the Doctor and Romana had defeated the Black Guardian, they were aware he was looking for revenge (hence fitting a randomiser to the TARDIS) and his servants would be looking out for them - with descriptions of their appearance. Hence changing appearance could make that vital difference between being detected or not. I feel the Key To Time season should have ended with both the Doctor and Romana regenerating to make it slightly harder for the Black Guardian to track them down.
If the new Doctor appears in August next year, does that mean there is a regeneration at the end of the Christmas special?
In the final episode of season 33, The Name of the Doctor we were introduced to another Doctor, played by John Hurt. But where does he fit in? There are two regenerations we never saw on-screen, namely from Patrick Troughton to Jon Pertwee and from Paul McGann to Christopher Eccleston.
Hurt being the third Doctor is hard to fit into what we have seen on screen - at the end of The War Games it was clear that the Doctor was about to start his sentence of a regeneration and exile to Earth. Some fans have come up with the "Season 6B" theory to explain continuity problems in The Two Doctors. After all, if Gallifrey had an advanced judicial system, then there must somewhere be a Court of Appeal which perhaps overturned the punishment on Jamie McCrimmon (and so got his memory back) and put the Doctor on a sort-of bail (movement restricted by perhaps the Time Lords remotely controlling the TARDIS, just as they did for some of the Pertwee adventures like Colony in Space and The Curse of Peladon until the Court of Appeal decided that the sentence of regeneration-and-exile was fair and was to be carried out - at the same time ruling that Jamie's memory was to be wiped and him sent back to Scotland).
However, it is possible that somewhere there was a "Season 6C" with Troughton regenerating (by choice) into Hurt, stealing a different TARDIS and then being brought back and forced to regenerate into Pertwee.
If that's not the case then this leaves the gap between McGann and Eccleston, and the logical assumption that the action that Hurt took without choice in the name of sanity and peace was the ending of the Time War by destroying Gallifrey. Nah, too logical and obvious.
A third possibility is that Hurt is playing the original Doctor, before William Hartnell, and that he did something so bad that he regenerated and fled Gallifrey. So bad that even in The Three Doctors the Time Lords dare not bring that Doctor back? So bad that the first Doctor's "There are five of me now" in The Five Doctors is to reinforce what he has been trying to tell himself - that he is the first - and making sure that the fifth Doctor sticks to that?
However we look at it, Smith is the twelfth Doctor. And it has been established that the Doctor can only regenerate twelve times. When Smith regenerates, the next Doctor is the last one?
We have already seen that rule overcome, in Utopia when the Master regenerates from Derek Jacobi to John Simm. For long-term fans there is a slight problem here, as one factor behind the Master's appearances from The Deadly Assassin onwards is his quest to get a new body as he has used up all his regenerations, ultimately stealing Tremas' body in The Keeper of Traken.
But if he has used up all his regenerations, how can he regenerate again?
I think the answer lies in the Chamaeleon Arch, which he must have used to change his DNA from Time Lord to human. When he reverted to Time Lord, then perhaps that reset his regenerations.
The Doctor used a Chamaeleon Arch to do the same in Human Nature/The Family of Blood - so maybe that reset him so that Tennant is the first Doctor and Smith the second.