Well we've had battle boner prompts before where WS loves his missions and gets turned on during combat. I was thinking it might be fun to have post CATWS Bucky avoiding Steve because he realizes that Steve doesn't like to kill people and that Bucky enjoying this would be a problem. I'd prefer Bucky to only go after Hydra or other 'bad guys' but you can make it all morally ambiguous up the wazoo and have victims that 'aren't that bad' in relative terms of being Hydra agents.
I'd also love to see a defector from Hydra joining him. Maybe one of the agents who used to be on the STRIKE team like Rumlow or Rollins. And of course, they have post combat sex all sweaty and covered in blood.