According to sources, one of either Colin Baker, Paul McGann, Tom Baker, Sylvester McCoy or Peter Davison will be in the feature length special episode alongside Tennant’s Doctor and the current incumbent Matt Smith.

Radio Times

I wouldn’t have posted that if it hadn’t come from the Radio Times. I’ve seen the rumour before but it sounded so outlandish. Now, as far as I can see the only reason to include Paul McGann in a list of ‘possible guest stars’ is that the guest star is Paul McGann. If you wanted to throw some possibilities in there to disguise the truth you wouldn’t pick the guy who had one movie.

It also makes sense that McGann’s ‘lost era’, the time between his movie and the emergence of Ecclestone’s Doctor in the new series is the Time War which we now know will feature prominently in the 50th.

Add this to the adamant assurance from some members of the cast that this is a story about three Doctors and three only then I’m forced to go all-in on my theory.

Three Doctors. Four Actors. Paul McGann and John Hurt both portray the Eighth Doctor at different points in his life.

(This is as much wishful thinking as prediction.)