The question that immediately jumps to my mind is: okay, is this your standard canon divergence AU where Steve was the one who fell off the train?
Or is this a universe where Bucky!Cap happens for the same reason as in 616? (Civil War ends in Steve dying, for Marvel values of "dying," none of which should be construed as implying anything about Cap 3 one way or the other.) Bucky takes up the shield, shoulders the burden, tries to atone for his Winter Soldier past and be worthy of Steve's memory... and then OH WAIT.
Re: WS!Steve / Bucky!Cap non-con
Or is this a universe where Bucky!Cap happens for the same reason as in 616? (Civil War ends in Steve dying, for Marvel values of "dying," none of which should be construed as implying anything about Cap 3 one way or the other.) Bucky takes up the shield, shoulders the burden, tries to atone for his Winter Soldier past and be worthy of Steve's memory... and then OH WAIT.