It was on AO3, set post-Avengers. Loki has been sentenced to live on Earth and is living in the Avengers tower. He spends most of his time in his room, but creeps out one day and overhears Thor talking about him with Tony.
Then he's out on the balcony watching a thunderstorm approach; I think Natasha comes out to threaten him if he misbehaves.
Then Thor comes out and says he just wants to see Loki smile again. It was this bit that was particularly touching, but for the life of me I cannot remember the name of the story.