I believe it’s a longer (20k+) mission fic set post-avengers. I can barely remember the overall plot, but I believe it ends with a town nearly freezing over, and Steve is caught in the crossfire and starts to freeze to death. Loki gives him a lifelong dream where he never crashes the plane (or does and is found very soon), and marries Peggy and has a family. In the dream, Steve realizes he isn’t aging as fast as Peggy, and she eventually dies of old age, while he lives up to the 21st century and joins SHIELD. Eventually the ice dissipates and Steve wakes up.
I’m sure is was a Gen fic, and the other avengers were there, but cannot for the life of me remember anything else about it.