I keep thinking about the scenes at the wedding where Sherlock was standing with John and Mary for the photographs and the photographer had to tell him to move, that he only wanted the bride and groom and the scene with the dancing, where Sherlock tells them to go dance and Mary says "What about you?" and John says, "We can't all three dance, there are limits."

What if there weren't any limits, and Sherlock showed up on their wedding night? Up to you whether he's there for a case or other reasons. Bonus points if it's Mary that wants him to stay and talks John into it.