Post-TWS, Bucky leaves Steve on the riverbank and disappears. And doesn't turn up on Steve's doorstep to sleep on his couch. Instead, he slips into the shadows to regroup and piece together his fractured memories, timeskipped slices of familiarity with the modern world, and deep-but-narrow insider knowledge of Hydra. Post-WS Bucky as a hardened, bitter assassin with a lethal skillset and a growing understanding of how badly he's been fucked over. To maintain his skills and contacts--and finances--he starts selectively freelancing as a hitman or bodyguard, often for shady underworld figures with Hydra connections he can glean intel from.
Okay, that's a lot of backstory for what I really want, which is this: Steve and Winter Soldier!Bucky nominally on opposite sides, beating the everliving crap out of each other on the job (and sometimes aiding and abetting each other on the sly), and cornering each other in their spare moments for emotionally-charged confrontations that lead to aggressive sex. Metal arm choking, kissing each other bloody, fighting-to-fucking, wrestling over who's on top, slamming each other up against rusted-out shipping containers or the walls of disused maintenance tunnels, trysts with a strict time limit based on when the explosives are set to go off, Bucky cornering Steve to go "who the hell was I to you?", Steve cornering Bucky to go "why are you working for these scumbags and why do you keep running away from me?", occasional furtive cooperation leading to classics like "okay, I'll let you go, but this has to look realistic--hit me in the face as hard as you can."
Basically, lots of violence kink. Super-soldiers fucking while bruised, battered, burned, and bleeding, and leaving each other in even worse shape by the time they're done. Penetrative violence (knives, bullet wounds) is a plus.
"Too bad," Bucky said as he pushed Steve up against the wall of the supply closet. "I should bend you over and fuck you right here." He shoved a knee between Steve's thighs and ground up against him, the armour plates in his combat suit hard and unyielding.
"I," Steve gulped. "Actually, you could, ah, fuck."