As it must happen:
My prediction. He will choose to become the Jailor's servant willingly, to spare Sylvanas from forcing it upon him.
They will kiss. They will separate.
He will look her in the eyes, and then bind himself to the Jailer, and cease to be Anduin.
Sylvanas will retain her hope and betray the Jailer.
How the Little Lion will make Windrunner the Hopebringer: