6. Acknowledge And Celebrate The Past 50 Years
One thing that The Three Doctors got very right (and goodness, was that already forty years ago?) was that it almost entirely existed as a tribute and love note to what had come before. Not just William Hartnell and Patrick Troughton, but also the wandering days, and the history of the Character. And what's more, they tied story into theme and made the return to the very earliest days of history to whole focus of the story. Of course, it was the earliest days of history that threatened to destroy the entirety of the universe, so it's perhaps best not to overthink this particular thread...