Tomorrow is the day we find out who the Thirteenth Doctor will be and so that means today is the last chance to get in some frivolous and self-indulgent fantasy casting done.
So, who would my pick be? Well, I think its time we had a bit of an upgrade, as Missy would say, and go from Time Lord...
… to queen. Yes, in my fantasy cast, Lena Headey IS the Doctor.
Why? Well, if there's one thing we know Headey can do its play a queen. She plays a queen in everything. She plays a queen in things where queens should not exist like Dredd. She has an aura of effortless authority that I imagine would be well-deployed with a properly Doctor-y speech. Just imagine her standing in the middle of Stone Henge delivering Matt Smith's “no plan, no back up, no weapons worth a damn” speech from The Pandorica Opens.
As to personality: I imagine her playing a rather more outwardly serious version of the Doctor akin to Capaldi's first season or later McCoy. Its all an act, of course, and once she's comfortable around someone that huge, wide open smile comes out.
I consciously chose a picture of Headey in a suit because... do I need a reason to appreciate Lena Headey in a suit? Anyway, I imagine the Doctor pretty much continuing with their previous fashion sense and coming to experiment with more feminine fashion over time, starting with some undercover business when they have to be inconspicuous at a fashionable party or such (and failing, obviously). On a practical level, keeping with “men's” fashion allows more of a visual through-line to their previous incarnations whilst on a self-indulgent level Lena Headey looks absolutely gorgeous in a suit (as the vast majority of women do, let's be honest).
It'll be another white guy, I just know it, but for one last moment I can imagine it. I can imagine Lena Headey in a suit dancing around the TARDIS console offering some young woman the choice of where to do (the companion should continue to be female even with a female Doctor, I just feel that should be said): the Roman Empire, the first Jovian colony; the Boston Tea Party; the Rolling Stones' first tour.
I imagine, for some reason, the Thirteenth Doctor being really into their classic rock and pop. I rather like that throwaway mention that the Tenth Doctor is a bit of a punk in Tooth and Claw and I'd like to revisit that idea.
But its going to be another white guy and we'll find that out after Wimbledon tomorrow.