Disclaimer: I don't own His Dark Materials or any of the characters. They belong to Philip Pullman.
Will stood in a glorious haven of light, and marvelled at the simple beauty and serenity of it. He wondered where he was, and then he realised that Lyra must have succeeded. She must have built the Republic of Heaven. Then he must be-
"Dead," he murmured to no one in particular. There wasn't even anyone there. Even Kirjava wasn't there. He mourned his own life for a moment. Then he heard a voice gently utter his name. He knew who it was. He would recognise those soft tones anywhere.
"Lyra?" He whispered, hardly daring to believe it. And then he saw her, standing a few feet from him, smiling, rapturous, beautiful, his Lyra, standing there in front of him.
They stood there for seconds, minutes, they didn't know. Time was meaningless in heaven. They simply stood there, eyes locked on each other, memorising each other's faces. Will wanted so much to hold her, to touch her, to kiss her, yet he couldn't seem to make his legs budge. So Lyra moved instead.
She sprinted across to him as fast she could, and he caught her and held her as tight as he could. They stayed in their embrace for a long time, kissing, laughing and crying with joy. And suddenly, they were twelve again, searching for their dæmons in the plains, feeding each other fruit, and revelling in the pure, innocent bliss of newly discovered first love. Once again they were sitting in those vast endless plains. Once again their kisses were desperate and clumsy. Once again their faces were hot, their hair smelt of honey and their mouths tasted of the little red fruit.
They whispered to each other sweet declarations of love, and they lied together under the tree they lied under a lifetime ago. They told each other their own stories and met loved ones from before. They spent every hour of every day together, making up for all the time they lost, spending eternity in that one day when they finally grew up.