So if Capaldi is retiring his mantle as The Doctor, the inevitable question is “Who’s next?”
A lot of clamor has been heard for a female Doctor, as has been proven possible by other Time Lords. And that, exactly, is the reason the Doctor should NOT be female in his next regeneration.
Missy only reveals herself as The Master to the Twelfth Doctor, Capaldi. This means that, while Smith’s Eleven interacted with Missy, he never knew his long standing arch nemesis had taken a female form this time around, and still believed The Master was well and gone, trapped in the time-locked Gallifrey.
The Doctor and The Master have been in competition since they were children, as friends often are. Much of their animosity seems to stem from this connection, a tie that The Master uses and abuses, and The Doctor fights so hard against. So why would The Doctor, who works so hard to separate himself from anything The Master does and is, be the copycat? After just finding out that The Master is a woman, how would he let himself regenerate in the same template?
The Doctor obviously doesn’t have complete control over his regeneration usually, though this concept is hotly contested. So it’s possible the gender won’t be his imitation of The Master, but it will always read that way, at least to the two of them, to The Master as a victory to lord over The Doctor, and to The Doctor as a failing of the subconscious impulses that shape his next form.
Go ahead, make him ginger finally, give him that dream… but this regeneration, whatever they do, I hope they don’t make him a xeroxed copy of The Master in making him a woman.
Sure, someday The Doctor should be female, just not this time.