Eurus waits in her cell.
She waits every week because she knows he will always reach out to her.
Week by week they plot in ten minute sessions. They're always recorded but what does that matter when only two people know the code.
Their song increases in pitch and speed week by week.
A year passes, the outside world communicated through song.
She smiles and she knows it is because she is happy. She plays this happiness with him and they deduce together what must be done for everything to fall into place. She can play faster than he can but she slows down for him, letting her mind take its time, spreading from one note to the next. She fills each swipe of the bow with emotion and things and places and people.
He smiles and she knows it is because he is happy. The music laughs as they plan.
Someday the East Wind would return to England again. It did not matter whether the government thought it a good idea or not- after all- who can control the skies?
This time there would be no storms, no flooding and no fire.
For that ending Eurus plays a song of hope and plausibility.
For that ending Sherlock plays with his little sister.