My prediction:

When the Doctor went back to 1953 in “The Wire”, he established an alternate timeline. Matt Smith’s Doctor is actually the second Doctor in his timeline, portrayed by Patrick Troughton in the original from 1966-1969.

The Doctor’s nemesis, the Master, is of course not dead. The Master in Matt Smith’s alternate-Troughton-timeline will be played by…David Tenant. But, to emphasize that he’s not the Tenth Doctor, he will have a sporty beard to rival Roger Delgado’s, or maybe just a goatee to show he’s evil.

Hey, it worked for Star Trek.