This probably would work best with female!Bucky, but mpreg is also cool.
Bucky runs on saving people. At their most sadistic HYDRA couldn't make him kill a child. So, the moment someone decided they would get him/her pregnant (for... reasons? Creepy cult-like resurgence around 2012, Bucky being naturally predisposed to the supersoldier serum - what with being Zola's only surviving subject) everything comes crashing down, because there's a lot of things Bucky will accept, but being hurt while there's a baby inside him/her is out of question.
The programming keeps, more or less, but Bucky leaves HYDRA (what's left or wherever's s/he've been kept at the time), midway through the pregnancy. Without freezing and wipes her/his memory starts coming back, just in time for the battle of New York.
Steve/Bucky (optional) HYDRA is undone by Bucky's pregnancy
Bucky runs on saving people. At their most sadistic HYDRA couldn't make him kill a child. So, the moment someone decided they would get him/her pregnant (for... reasons? Creepy cult-like resurgence around 2012, Bucky being naturally predisposed to the supersoldier serum - what with being Zola's only surviving subject) everything comes crashing down, because there's a lot of things Bucky will accept, but being hurt while there's a baby inside him/her is out of question.
The programming keeps, more or less, but Bucky leaves HYDRA (what's left or wherever's s/he've been kept at the time), midway through the pregnancy. Without freezing and wipes her/his memory starts coming back, just in time for the battle of New York.