9. A Christmas Special Based On A Non-Canon Christmas Story
Under Steven Moffat, the Doctor Who Christmas specials have often been reworkings of classic Christmas stories. Some of them are good (A Christmas Carol), some of them are bad (The Doctor, The Widow and the Wardrobe), but they remain an integral part of the modern Doctor Who experience. While Gatiss and Moffat have openly said that every Holmes adaptation can be considered canon (and therefore can be borrowed from), it would be interesting to see how they would deal with branching out from the Doyle universe. There is a lot of humour to be mined from the contrast between traditionally sentimental Christmas stories and Sherlock himself, who once described sentiment as "a chemical defect found in the losing side." There's no need to slavishly follow the story of, say, A Christmas Carol, just as the series currently cherry-picks from many different stories for a given episode. But with a good idea at the heart of it, we could have a Christmas special as hilarious and subversive as Blackadder's Christmas Carol.