The Prophecy in 'The Day of the Doctor'
When it was announced that David Tennant would be returning to Doctor Who, it was a surprise to everyone, but the 50th-anniversary special predicted exactly this would happen a full decade ago.
Doctor Who Tom Baker Day of the Doctor
David Tennant's run as the Tenth Doctor was beloved, with his performance catapulting the new era of the rebooted Doctor Who into a new level of popularity among an entirely new fan base. His regeneration and departure from the show was hard on audiences, who had grown attached to his version of the Doctor over the seasons he piloted the TARDIS.
Tenth Doctor fans were delighted, then, when Tennant returned as Ten for the Doctor Who 50th-anniversary special episode, 'The Day of the Doctor.' It brought together Tennant's Tenth Doctor and Matt Smith's Eleventh for a rollicking adventure. However, one specific scene in that episode predicted Tennant's recent return as the Fourteenth Doctor, ten years before it even happened. Even better, it was a prophecy handed down by the Fourth Doctor.
The Curator's Cheeky Prediction
One scene in 'The Day of the Doctor' took place in the National Gallery museum in London. The Tenth and Eleventh Doctors headed there to stop Zygons from emerging through the paintings into the modern day, but Eleven met another familiar face while there. The Eleventh Doctor stops to speak with the museum curator, and the Curator is played by none other than Doctor Who's own Tom Baker, who played the fourth incarnation of the Doctor. The Eleventh Doctor is surprised and remarks that the Curator looks exactly like one of his old incarnations.
It's at that moment that the Curator makes a cheeky prediction. He seems completely unsurprised by the Doctor's words and explains that in the future, the Doctor will regenerate into previous incarnations, but offers the caveat that it will be 'just the old favorites.' The scene seemed to indicate that in the future, an incarnation of the Doctor would figure out how to control their regeneration and that the Curator was, in fact, a future Doctor who had regenerated with the form of the Fourth Doctor and was now living as the Curator of the National Gallery.
David Tennant's Doctor Who Return Is A Confirmation Of The Curator's Hints
David Tennant's return as the Fourteenth Doctor, however, turns the Curator's words from merely a fun meta-reference into a prescient prediction. Technically, it's not the first time the Doctor has regenerated into a previous incarnation. During Tennant's own run, in the season 4 episode 'The Stolen Earth,' the Tenth Doctor was shot by a Dalek and forced to regenerate. However, Ten forced his regeneration energy into a hand he'd previously severed, allowing him to regenerate as himself and intact with his same personality. While it was later confirmed to technically be an official regeneration, it was more like a minor speed bump in Ten's timeline and doesn't really fulfill the promise of the Curator.
The regeneration of Jodie Whittaker's Thirteenth Doctor into David Tennant's Fourteenth Doctor is exactly what the Curator hinted at, however. It's not just a restart of a current Doctor, but an entire reboot. While Tennant has cautioned that the Fourteenth Doctor is not the same as the Tenth Doctor, that's more a matter of him having experienced three different incarnations in between rather than him being completely new and alien. Fourteen is still essentially Ten, very much one of 'the old favorites,' just with a few more miles on his soul. It's not yet been explained why the old incarnation returned. But if anyone in Doctor Who could figure out how to control the mystery of regeneration, it's the Tenth Doctor.
David Tennant as the Fourteenth Doctor holding the screwdriver in Doctor Who 60th Anniversary Teaser Poster