Predictably, a million entitled, stupid, misogynistic manbabies are shitting all over the Internet at the news that the next Doctor is going to be played by a woman. Equally predictably, there are some good jokes at the expense of said manbabies doing the rounds. Of all the iconic well-known and well-loved characters in popular culture, there has never been anyone more suitable to be played by any gender, ethnicity, orientation or ability level than the Doctor, FFS! This is a character who changes into a completely different person, every so often – not just a different outfit, or a change of hairstyle, but a total remodel. So, while you might have a personal preference for one actor over another (and might, indeed, have lost interest in Dr Who a couple of years back when the storylines got more and more pathetic) there is no justification whatsoever for claiming that it is impossible for a woman to take on the role. None. The feasibility of the idea is all over the canon (according to friends who have watched rather more of it than I have, anyway). There is no big leap of internal logic required, and no more suspension of disbelief than is necessary either to accept the idea of time travel or to put up with the occasional thumping great plot hole.
It’s hardly unusual for writers to take on any iconic character/story and do a version where the genders – or ethnic groups, or social class, or even age – have been changed: it’s a concept that’s almost a cliche, these days. Sometimes people simply take an old, familiar story and rewrite the core of it in another setting; sometimes there’s a closer and more openly-acknowledged connection, such as making the new protagonist a descendant of the original. Some of these work better than others.
For the erotica writer, it can also be interesting to flip the genders of your characters and see what that might do to the story. (OK, if you are writing to a CFS which has specified M/M or F/F stories only, now is not the time.) If your story has got bogged down somewhere, or you have a nasty suspicion that it’s too much like every other story in the genre you ever read, take a look at the genders you have assigned and ask yourself what difference it would make if even just one of them was different. If all that comes to mind are stereotypes, look at the impact of these types on the story you are telling, and then fuck around with them some more.
Maybe make one of them into a time-travelling alien while you’re about it.