WAIT I thought of something even better.
So when Steve wakes up, Peggy tells him that Barnes went missing in the early 60s - just disappeared without a trace. Most people think he died, but Peggy thinks he just wanted to live out the rest of his life in peace. She says maybe he’ll come out of the woodwork now that Steve’s back, which is terrifying to Steve because he doesn’t know how he would handle seeing Peggy and Bucky old and tired and ailing. But some years pass, he moves to DC, and there’s never any sign of Bucky. Steve doesn’t think he’s a live because - if he were - he would have come by now.
Meanwhile, back in the 60s, Bucky kept looking in the mirror and realizing he looked like he’d aged maybe five years while everyone else was getting older. Shield started giving him shady questions about it (house rules whether he has the arm or not; it could probably go either way? I kinda like the arm. I kinda like the idea of him crawling back to camp minus a limb and everyone being horrified and awed) and Bucky doesn’t want to become Shield’s lab rat so he goes off the grid. For the rest of the century he lives a pretty quiet life, traipsing around the middle of nowhere, keeping an eye on Shield from the outside. When Hydra starts to take over, he sees that, too, and does what he can to slow the process, obviously with little success.
Hydra’s aware that someone is interfering, but they don’t know who. Still, Bucky’s not going to show himself for some guy “Shield” is claiming to be Steve Rogers. Bucky does watch him closely, mostly convinced that it’s Steve, but still mistrusting of how Hydra might be using him. He is leading a Hydra strike team, after all - Steve could be brainwashed for all Bucky knows. So he waits to make his move.
And then we’d probably have a similar string of events to CATWS, but with a happier (??) backstory? Bucky shoots Fury to get him out of Hydra’s hands. Maybe at the end Steve is about to go down with the helicarriers and Bucky finally reveals himself to save him.