Russell T. Davies has revealed that not only has the Thirteenth Doctor been cast, but he also knows who it is!
As a former showrunner, Davies is often asked about what he thinks about the show now. But this time, with Peter Capaldi leaving at Christmas, RTD was questioned about there being a female Doctor. While he was non-committal about a female incarnation, after a bit of discussion, he revealed:

“I do know who it is.”

Six words to set fandom alight. No, not “don’t you think she looks tired?”
This is the first time we’ve heard that the Thirteenth Doctor has indeed been cast, although many have suspected it. Indeed, some are adamant said actor will be in Series 10, however unlikely that sounds right now.
The interview, by Andy West, took place at the LGBT Awards, and talk of Doctor Who begins at 3:27:

He also said:

“Having worked on it, I am leaving it to them. And any time you pass any sort of comment on Doctor Who, they go mad.”

Now, why would you say that, Russell…?
This is big news. Like, big. This is, of course, assuming Davies wasn’t simply winding Andy up. Always possible.
However, Davies would be just the guy who would know – having worked with both Steven Moffat and Chris Chibnall, and having been in a position of power, he’d likely have been told. Right?
We can only hope this means an announcement is soon. Pretty please?