In a recent interview with Daily Record about his newly acquired OBE, Moffat casually revealed:
"Ahead of me this year, I've got fourteen Doctor Who episodes and three Sherlock films."
It seems unlikely that this is just a mere slip of the tongue, as Moffat previously told Digital Spy:
"I know that what I've signed on to do is 13 more episodes of Doctor Who - including a Christmas special. I'm trying to argue for slightly more than that, but we'll see."
With Moffat's departure from the show, it would be fitting for him to leave us with an extra surprise. There has been no mention of whether this is an extra episode or a one-off special, but as ever, only time will tell.