Given that both of Moffat's Christmas specials so far have been based on existing works ("A Christmas Carol" and "The Doctor, the Widow, and the Wardrobe"), I wonder if this Christmas Special will follow suit when introducing this new companion.
If so, I think the best existing Christmas work to get a Doctor Who re-imagining would be "It's a Wonderful Life". Series 5 and 6 were all about all the damage the Doctor has done and how much the universe has turned against him, and with him losing the Ponds in a "heartbreaking" manner in the episode prior to the special, I think it would be awesome to send him on a trip through time to see all the good he's done, and what the universe would be like without him (I know "Turn Left" already kinda did this, but the Doctor wasn't there to see any of it, and Donna didn't realize it was wrong). And somewhere along the way he picks up new girl. Hell, maybe she's the guardian angel.