(What I wish would've happened in the Christmas special. Alas.)
When the Doctor closed his eyes, he knew everything had changed. Now, after an eternity condensed into a moment, she wasn't sure she wanted to open them again.
They saved the day, in the end. They put things right. Fixed the timelines, her and her first self. But at what cost? The Master. Missy. Nardole. Bill. Her mind went further and further back, as it tended to do whenever she found herself in this situation. But this time, all she could think of was the ones she left behind. The ones she failed. Clara. Amy and Rory. Lucy. Tamsin. Adric. Katarina. Sara. The list just kept getting longer with each new life. Was there ever someone the Doctor didn't let down?
As she lay there, the heavy silence of the icy landscape around her was interrupted by the crunch of boots through snow. Exhausted, the Doctor remained motionless, uncaring. Her interest was only peaked when the footsteps stopped right in front of her. Slowly, she managed to push herself up slightly and open her eyes. The face looking back at her was one that stole her breath away. The fond smile that slowly spread across her face in reaction had never been more genuine.
"Hello, Grandmother. It's so very wonderful to see you again."
"...Susan, my dear. Oh, how I've missed you."