This rocks a thousand times. I liked the flow of the words, the repetitions that sounded like the incantation for a Dark spell, and all the implications you draw from the epilogue. Especially the possible parallel with the Dumbledore family was brilliant, I haven't seen that at all. And I loved this passage to little pieces:

Only in the places where the tracks cross, the lines meet, the threads tie, only here can you all exist for each other. Here in these intersecting moments, you make each other who you are; you make each other real.

Draco Malfoy only exists for you in a train station, autumn to autumn, Christmas to Christmas. Gindelwald only rises when Dumbledore shakes his hand; Tom Riddle only succeeds because there’s no one here to meet him; Severus Snape’s driving force in life was Lily Potter’s eyes, and Rosie will smugly outshine Scorpius just because her father told her to.

For a moment you see the future folding out before your eyes, as the past has just done, and you see the two are mirrors. They are the same. And even if the scar has not hurt you for nineteen years, fear, blatant and sudden, seizes you. It makes you crouch down, so that your son’s face is slightly above your own, so you can whisper away your fears like a dirty secret.

It's chilling; very well done! And thank you for sharing.