Steven Moffat himself clearly knew the talent so close at hand, too, and so even after that glorious final finale moment, we’ll be once again seeing the original Doctor teaming up with the Twelfth to bring the story to a close.
There’s no way that you cast an actor like David Bradley as actor William Hartnell, as well as The First Doctor, without finding a way to use him in the official canon.
His performance in the TV biopic An Adventure in Space and Time was such a winning take on for the classic actor, as was his incarnation of the very first Time Lord on Doctor Who.