Now here's an idea, how about David Tennant, anyone have any ideas what he's up to for the next 12 months stage and screen wise? I know it is unlikely and I know there are many who'd groan at the suggestion, but it would be a change to bring back a previous Doctor and could fuel some good story ideas.
Who knows what consequences would ensue from the Doctor regenerating (or should that be degenerating) into a past incarnation? It could be what sparks the creation of the Valyard.
Also the secrecy thing as to the identity of the new Doctor could be maintained, at least until filming on Season 8 starts. The regeneration scene could have been filmed for the Anniversary special and if not, Tennant's presence in costume during the filming of the Xmas episode could be explained away as re-shoots or 'pick-ups' for the Anniversary show.
I can just see the scene now as the end of the Xmas special, Matt Smith changes back to David Tennant who staggers over to mirror in the throws of post-regeneration stress looks at a mirror and exclaims "What? WHAT?"