Theory for New Companion

Moffat is not telling us the name of the new companion, which probably means she's a returning character. If she's being played by a new actor, then she must be a Time Lady who's regenerated. The only Time Ladies we've met are Susan, Romana, and Jenny (who doesn't have proper regenerative capabilities). If it's Romana or Susan, they might have been hiding in the TARDIS' cryogenic chamber (Moffat did say she'd be in the last place the Doctor would ever suspect, which could be his own TARDIS). After being in there for two centuries, her brain might have been addled and she'd be unable to remember how she got there. If the new companion was Jenny, she could have undergone a proper regeneration later on. She then could have been captured by the Daleks and held for interrogation on Skaro (the Doctor wouldn't expect to find a friend there, would he?). The Doctor would then have to come rescue her. Her interrogation by the Daleks could have damaged her brain, leading her to forget how she first came to be captured. The only problem with the Jenny theory (other than suddenly being able to regenerate properly) is that it doesn't seem very Christmas-y. Then again, all the Christmas specials whose plots haven't been copied from a book haven't been very Christmas-y.