If you like a great gen fic with a canon feel, I highly recommend "Adagio Lamentoso" by Vrazdova. I thought the author's writing was just splendid, feeling right for both ACD's Watson and BBC's John, their voices merged. John and Sherlock are wonderfully in character and I really enjoyed the portrayal of their friendship.
There are some dark themes, so you might want to check the tags - and then I hope you enjoy!

Link to AO3 (http://archiveofourown.org/works/372994/chapters/607945)