A Dead Man At The Door
Sherlock is gone and John eventually moves on with his life. No sense waiting around for a dead man. He gets married because that is what one does at his age. He can't hold on to Sherlock forever.
But the thought of the dark haired man still brings tears to John's eyes when he thinks no one is looking. But someone is always watching.
And John's fiance knows that he will never love her the way that he loved, no loves, Sherlock.
On John's wedding day he glances over his bride's shoulder to see a very familiar dark haired man sitting in the back pew. When he glances back, a few seconds later, he sees no one.
John lives the next year of his life always feeling like something, someone, was missing. Everywhere he goes he thinks he sees the taller man in the corner of his eye. It never is. John shakes it off as going nuts. He begins to ignore it.
Then one rainy day, when John's wife is out for lunch with her sister, he hears a knock on the door. When John opens the door, Sherlock is standing there.
"John." The man says casually, as though they had seen each other that very morning.
John stands there, gasping wordlessly, as Sherlock pushes his way past John and into the man's home.
And so it begins. The dark haired man pulling the light haired into adventures. As though nothing had happened.
But something had. John was not the same as he once was. He had grown up. He had given his love to another. No longer were they "John and Sherlock the couple" now they were "Dr. John Watson, and Sherlock Holmes the consulting detective: Partners in crime".
They were not the same as they once were. No dark to balance the light, no light to balance the dark. No longer one.