What if the entire show is actually from Mycroft’s memory?
After Sherlock is killed in the final confrontation with Professor Moriarty (Jim’s brother), Mycroft steps down from his government job and moves into the country near his parents, living a solitary life, blaming himself. When he is older, John and Mary’s grown daughter seeks him out and asks to hear the stories of Sherlock Holmes.
The final scene is the Watson daughter laying her hand over his.
She smiles softly and says, ‘They never blamed you.’
For the first time in twenty-five years, Mycroft manages a small smile and the burden on his heart is eased just a little bit.
And as the Watson daughter walks out to her car, she hears music coming from the house and smiles. Through the window, we can see Mycroft playing Sherlock’s violin with a sad smile on his face.