I feel like there aren’t enough AUs where Bucky doesn’t fall off the train and he and Peggy sit and watch together as Steve sacrifices himself and he helps her form S.H.I.E.L.D and Peggy and Bucky become best friends but every once in a while when they’re talking they pause, almost waiting for Steve to pipe up before they remember, and they never stop missing him. It’s Peggy who Bucky comes to first when he realizes whatever Zola did to him way back when Steve had to rescued him has stopped him from aging, it’s Bucky who has to slowly watch as Peggy gets older and her memory starts to fade.
And Bucky continues to work at Shield, and when they finally find Steve, Bucky is there and it just takes his breath away for a moment to see him there, alive because he’d never thought he’d see him again. And instead of the nurse sitting there when Steve wakes up it’s Bucky, and the radio isn’t playing because “We went to that game together, he’ll know something’s up,” and at first Steve doesn’t notice anything different when he wakes up but then he realizes Bucky is crying and immediately after Bucky explains and Steve’s heart just breaks.
Imagine the joy in both of them when they realize neither of them going to get old. They go visit Peggy together every week, and every once in a while they’ll stop by Steve’s Smithsonian exhibit just to point out all the inconstancies