With the news that Matt Smith is to leave the role of The Doctor (let's call him Eleven from here on in) at the end of the show's current 50th year, Doctor Who's producers have a crucial decision to make in appointing the right actor - or actress - to take on the role of Twelve.
I've been a lifelong fan of Doctor Who, although in my case I was still only four years old when Sylvester McCoy became Seven, so don't really have clear memories of any of the Doctors who came before him.
Before I list some of the options, let's remind ourselves of the Doctors of the past:
1. William Hartnell (1963-1966), 2yrs 11mths
2. Patrick Troughton (1966-1969), 2yrs 8mths
3. Jon Pertwee (1970-1974), 4yrs 5mths
4. Tom Baker (1974-1981), 6yrs 9mths
5. Peter Davison (1981-1984), 3yrs 0mths
6. Colin Baker (1984-1986), 2yrs 9mths
7. Sylvester McCoy (1987-1989), 2yrs 3mths
8. Paul McGann (May 27th 1996), 1 day
9. Christopher Eccleston (2005), 3mths
10. David Tennant (2005-2010), 4yrs 7mths
11. Matt Smith (2010-2013), 3yrs 11mths
Although it might feel like the actor playing The Doctor has changed quite a lot since the show's return in 2005, that's actually not the case - with the exception of Eccleston's admittedly short tenure, Tennant and Smith have been two of only four actors to play the part for more than three years (with Davison piloting the TARDIS for almost exactly three years, but falling just a few days short of the milestone).
The next Doctor, then, will be looking at playing the role of Twelve for around four years or more, if they are to hold their own in the history of the show.
It is perhaps also worth noting that The Doctor is rapidly running out of regenerations - the canon of the show generally has it that The Doctor has 12 regenerations at his disposal, plus his original incarnation, making a potential 13 different actors in total. No doubt, when it comes down to it, the writers will find an excuse to continue changing the lead actor into Fourteen and beyond, but I suspect for many fans, the first 13 Doctors will always hold a special significance.
So, who do we expect to take over the role? I'm going to try to avoid making too-specific suggestions, and instead list some more general categories from which Twelve is possibly going to be picked.
Since its return in 2005, Doctor Who has attracted a huge female audience, not least because everybody fancied the pants off of David Tennant. With that in mind, there's been a growing clamour for The Doctor to become female, each time he has regenerated since then.
I'd find it uncomfortable if Twelve were to be female - The Doctor has always been a 'he' and, although the regenerative process clearly allows for change, he's always taken the form of a human male up until now, and it would seem odd for that part to change abruptly, particularly in terms of a fairly major characteristic like gender.
Plus, to be frank, pretty much everybody hated River Song. Any actress taking on the role would have to work hard to overcome that; The Doctor is pretty ascerbic at times, and I think it's easier for a male actor to shrug off seeming smug, compared with Alex Kingston's annoyingly know-it-all portrayal of Song.
One thing that has been particularly prevalent in Doctor Who since its return is a willingness to have 'aliens' who are clearly just humans in costume. A great many of the alien species that The Doctor has encountered since 2005 have been fundamentally humanoid, and that potentially unlocks the door for The Doctor himself to be designed in such a way.
A Dalek-Doctor might be a stretch too far, but an Ood-Doctor wouldn't be far beyond the pale, provided that he only took the visible form of an Ood, and retained all of his usual dynamism.
Will the producers take such a gamble? Probably not - The Doctor's human side plays a pretty important part in allowing the audience to relate to the character, but it's certainly an outside possibility.
A Previous Character
Given the extreme focus the modern incarnation of the show puts on The Doctor's relationships with his companions, could it be feasible for the producers to bring back an actor who has already been on the show?
Could we see Twelve portrayed by John Barrowman, or Billie Piper, or Catherine Tate? All have been granted Time-Lord-like abilities at some point in the past (but then again, Amy Pond was given the ability to 'remember' the entire Universe back into existence, while Clara Oswald has just distributed herself throughout The Doctor's entire history, so there are plenty of supernatural human characters to choose from).
Of all these options, Billie Piper seems the most likely, as she was fundamental to the success of the revived series - but perhaps a little too long has already passed for the audience to welcome her back, particularly in the main role.
Although it pains me to say it, I'm increasingly expecting an outright comedic actor to be cast into the role. Or not an actor at all, but just a stand-up comedian.
More and more episodes of Doctor Who are described as "so much fun" in the reviews, and while the days of hiding behind the sofa may be gone either way, it's pretty painful to watch one of the world's most successful and cherished science fiction series descend into slapstick and one-liners.
But I'm not in charge; the producers are, and if they decide they want Russell Kane to play Twelve, then that's the way it will have to be.
For the past few regenerations, The Doctor has been disappointed not to be ginger... given their penchant for comedy, will the producers finally decide to pay off on this gag?
If they don't, I actually wouldn't object too much if they decided to throw in a ginger midway regenerative phase, just to provide a punchline to a setup several years in the making - they could always have him end up blonde without it being too much of a stretch of the imagination for him to go through a ginger phase partway through the process.
Nick Grimshaw. Radio 1's Breakfast Show host, BBC Three panel show host, Never Mind The Buzzcocks guest host, Children in Need host, Comic Relief host, Glastonbury host... Grimmy is frankly everywhere and, love him or hate him, they're gonna keep ramming him down our throats until we stop resisting.
Plus, with his 'quirky' hair and northern accent, he's annoyingly close to what the Beeb are likely to be looking for to replace Eleven. Let me be the first fan to plead: Don't do it. For the love of god.
OK, so if I'm going to discount all of the above, who would I personally like to see in the role?
Well, my vote goes to Chris Geere. Formerly of Waterloo Road and now appearing in After Earth, Chris has the range and the quirky personality, a pretty hardcore cult following of fans, yet is still unknown enough to many that he wouldn't seem too much like the show selling out to a 'big name'.
He gives great hugs, which seems like a pretty Doctory quality to me, and he'd be the blondest Doctor since... well, since the days of Davison and Colin Baker, at least. Just think of all those 'blonde' gags the writers could throw in without having to make any effort at all...
Seriously, this is not the US presidency, casting a woman or a black actor would not be making history, but it would be something of an indulgence at the cost of a well-established character. Do what you will with the rest of the show - and heaven knows it's seen plenty of rough times in the past - but the character of The Doctor himself needs to go to someone with the necessary aplomb to carry off both the comedy and the tragedy of the role, and to make both believable at once.
Who would you like to see as Twelve? Or would you prefer to see an entire regeneration used up on John Hurt in the 50th anniversary special, and move right on to Thirteen at Christmas?
Let us know your thoughts below - a female Doctor? More comedy? A big name? Or an unknown? Place your bets now...