Headcanon for Sherlock Series 3
John's about to have a row with the machine at the supermarket, when he realizes he's got no one to tell the story to at home, so he calmly presses all the buttons he needs to and doesn't leave in a huff at all.
We see Sherlock, obviously, but I don't think there will be any Sherlock/John interaction, at least not until the end of the episode. We'll see John coping, and Mrs. Hudson getting used to making only one cup of tea instead of two despite her not being their John's housekeeper, and we'll meet Mary The Girlfriend.
Sherlock, meanwhile, will be doing whatever consulting detectives do when everyone they love thinks they're dead. Except for Irene. He'll probably text her, and say things like "Must be a stupid girlfriend this time if you let her fix your shift key, I noticed your texts aren't all in caps" to which she will reply "Well should I have my strudel with or without the frosting for breakfast today?" etc etc.
And obviously Jim won't be dead, and he'll be shagging the deliciously and mysteriously handsome Sebastian, all the while exchanging flirty looks and possibly texts with Sherlock and bragging about how he survived a gunshot and Sherlock only survived jumping off a building, honestly Sherlock, anyone could fake that kind of suicide.
And all the while John would be smiling pleasantly at the girl who maybe works at the cash register at that supermarket, because that was one time when he didn't use the machine and she thought he was cute and he thought she was different enough from Sherlock while not being completely stupid that maybe it could work, and then suddenly they're going out. And then maybe he proposes to her. And maybe she says yes.
And then at the end of the episode, John comes home to 221B Baker Street, expecting to find an empty flat because Mrs. Hudson's out with her new husband or whatever it is now, and Mary - Mrs. Watson, oh, that sounds so fantastically normal, Mr. and Mrs. Watson - is out with her friends. So John comes home and Sherlock's sitting in John's chair, on his laptop, and says, offhandedly, over his shoulder: "Really, John? The married life?" And then it cuts to John's shocked face, and perhaps he drops his groceries.
AND THAT WOULD BE THE END OF THE EPISODE BECAUSE MOFFAT IS A TROLL.