My trashy brain would love a fic where the Winter Soldier was used as a test subject when the U.S. gov was experimenting with mental depatterning treatment to break/brainwash enemy soldiers. Or maybe it's right after CAWTSr and it's Hydra's last attempt to keep Bucky under their control.

So they do the whole nine yards: long term sensory deprivation, electro shock treatment, induced coma in order to break Bucky down to a completely clean slate. Bonus if the fic includes the re-mothering part of the process as they try to raise a perfect assassin.