In nearly ten years since Doctor Who's resurrection, three of the five still-living actors to play the title role before 2005 have (at least in some capacity) reprised their role onscreen. Peter Davison appeared as the Fifth Doctor, alongside future son-in-law David Tennant's Tenth, in the 2007 Children in Need minisode 'Time Crash'. And of course the Eighth Doctor got his long-overdue second TV turn, played by Paul McGann, in the prelude for the 50th anniversary special last year in 'Night of the Doctor'. People debate Tom Baker's appearance, since his enigmatic character - The Curator - was never explicitly stated to be the Doctor, though his brief exchange with the Eleventh Doctor left little doubt that this mysterious man either was, is or could eventually be the Doctor in some form. Otherwise, classic series representation in the anniversary special was lacking. So why not complete the set and bring in Colin Baker as the Sixth Doctor and Sylvester McCoy as the Seventh Doctor to give both incarnations a final bow. This might not work if Peter Capaldi's dark, irascible incarnation was suddenly sharing the TARDIS with a walking rainbow or another perpetually irked Scotsman, but the show is in a great position to make a tribute to the later days of classic Who without breaking continuity or the tone of the ongoing story. And here's exactly why...