Disclaimer: Sherlock and accompanying characters belong to BBC, etc. I'm just borrowing them briefly.
This little piece was inspired by The Sign of Three. While I tend towards shipping Johnlock, it turns out that I really like John and Mary and I wasn't expecting to.
Sherlock folded the sheet music into thirds and tucked it into an envelope for John and Mary. He left it in on the music stand and slipped quietly out of the gathering. He was done for the day. For John and for Mary, he had been willing to participate in the wedding and be reasonably polite; he was even honest without his typical accompanying acerbity. All that was left for the wedding was the dancing and then the Watsons' departure.
He had studied his duties as best man, so before he left the hotel grounds, he dutifully decorated the car John and Mary would be leaving in.
As he put the finishing touches to the "Just Married" sign on the car bumper, his mind wandered back to the deduction he'd made regarding Mary just before he had left. There would be three of them now; everyone had been telling him that he was no longer necessary to John now that he was married, and it would be doubly so now that he was about to become a father. And yet...
There would be another Watson in the world to watch over. After all, he had vowed to be there for them, no matter what. It was a promise he intended to keep. Three of them. Like the three beats in the measures of the waltz he had written for John and Mary. Three beats to the measure, written in quarter notes. Three notes, and the implicit fourth.
A smile broke across his face. No, the Watsons wouldn't be excluding him from their new lives. As he had said earlier, it was a new chapter.
And this chapter would include someone he could teach the science of deduction to from infancy.