Try this one on for size. River Song is able to de-age herself ('slowly at first, just to freak people out') even after regeneration. Of course, this is to explain Alex Kingston looking younger in previous episodes. Taking Allyn's It's a Wonderful Life analogy to its' conclusion, what if (in the just-shot surprise finale) Hurt's Doctor at the end decides to 'take the age down a bit,' take an electric razor to his beard and hair, and what emerges (via archival footage/CGI trickery) is Eccleston?
It'd be even better if he emerged as River Song.