4. A Farewell Tour
We won't see the year long farewell tour Tennant had. Smith only has one episode and it would be heartwarming for him to get a chance to say goodbye to his old friends. Rory and Amy are stuck in New York and it's highly unlikely we'll see them but couldn't he just meet them somewhere on the New Jersey Turnpike for a quick lunch? His wife River has presumably said her last goodbye already but I highly doubt we've seen the last of her character. She is part time lord remember and whether the character of River is played by Alex Kingston or not, don't count her out as a companion to future incarnations. That just leaves a handful of sidekicks. Madame Vastra, Jenny, Strax, Craig and young Stormageddon are just a few of his past associates that would be nice to see in Smith's final episode. IMDB only lists Smith, Coleman and Capaldi as the episode's cast, so there are many options and surprises that wait to be seen.