First off: I'm excited for Mary. I can't wait to see what she brings to the show. But I still think Sherlock would see The Wedding as a bit of betrayal, no matter how brilliant Mary is. So I wrote this. The title is (of course) from the wedding poem. I figured the "something old" was the friendship between Sherlock and John and the "something new" was Mary.
They're standing in a small side room, torturously waiting. Sherlock can hear the seating music wafting across the air and into their little sacred haven. This is the last time he'll have John all to himself. The last time it's Sherlock-and-John, not Sherlock and John, hatefully separated by spaces and spouses. John is jittery, checking his reflection in the mirror, straightening his tie, checking his collar. Sherlock can't look at him. He looks too happy. How can he look so happy when, in mere moments, he will give his life away to some... some.. woman and never see Sherlock again? Sherlock's breathing is getting panicky. Being John's best man is the cruelest thing he's ever had to do, save leaving John broken and alone on a sidewalk for three years. He can't stand here during his last moments with John and watch him avoid his eyes.
"I'm going to go check... I'll look and see... I'm..." There are no words. Sherlock darts for the door.
"Sherlock," John says, and it's warm, and it's affectionate, and it's all those things John is but soon won't be and it tears at Sherlock's heart.
"Yes?" he asks, and his voice is full and heavy.
"Come here, I have to... have to give you something."
Sherlock takes a deep breath. He doesn't know if he can look at John. He turns and comes back, eyes on the floor. John's feet shuffle back and forth, and they're standing barely half an arm's length away from each other.
"Sherlock, I know this is hard, and it's difficult for me, it really is. But I want you to know that I'm not abandoning you. I still want you in my life." There is a bit of a pause while John clears his throat. Probably wets his lips like he always does- Lord, he'll never see John wet his lips again.
"I want you to have these. As a promise." John roughly shoves a box into Sherlock's hands, like he's afraid if he does it too gently Sherlock won't take it. Sherlock just holds the box. He can't move.
"Will you open it?"
His voice has never been so ragged.
"Sherl..." John gently takes the box from his hands and opens it. Sherlock hears the jingle of a chain, but still doesn't look up. Then John closes the space between them and slips something cold and metal over his head. Sherlock looks up in surprise to see John four centimeters from his nose. Time expands for a moment while John takes in Sherlock's wet eyes and Sherlock tries to drown himself in the endless warmth and love in John's. Then Sherlock looks down. His breath hitches.
There, gently resting on his chest, are John's dog tags. The little circles brush together and make a small noise. Sherlock can't breathe.
"I want you to wear these. I don't want you to delete me from your "Mind Palace", or whatever." John tries to make his voice light, but fails. Sherlock is still looking at the tags. John reaches out and helps slide them underneath Sherlock's shirt and tie, hiding them from prying eyes. They are a cold pressure above Sherlock's heart. John grasps either side of Sherlock's head, forcing eye contact from the taller man.
"Listen. No matter what, no matter where I am or what I am doing, I want you to be a part of it. I want you with me. I want you in my life. Please, Sherl."
Sherlock grasps John's head and crushes their foreheads together. One or both are crying.
"I need you, I need you, I need you." Sherlock prays over and over, begging John to understand.
"I'm here, I'm here." John reassures, his fingers combing through Sherlock's curls.
They stand there, alone and together, foreheads the only point of contact, tears tracing tanned cheeks and hollow ones.
The knock on the door comes too soon and they pull apart slowly, each move physically paining them. John stands on his tiptoes and presses a long, dry, chaste kiss to Sherlock's forehead, a charm to ward off danger and a promise. Sherlock lets out an audible sob and clutches one of John's belt loops.
Both have dry eyes and smiles through the ceremony.