According to some sources, it's possible Avengers 4 might involve some kind of time travel fuckery to try to solve the Thanos mess. So, Steve and co. go back in time and while trying to accomplish their mission, happen upon the Winter Soldier mid trash party.
It can be as handwavey as you want, maybe they are trying to get HYDRA's information about the Tesseract, maybe the Asset is stationed somewhere with a team on a mission to kill Hank Pym/Peggy/Howard (perhaps he botched the mission so he is being pubished).
BONUS: + The person with Steve is Thor. +They can't interfere for fear of fucking up the past (although that's what they'd be trying to do? I don't know how time travel will work in the MCU if they really go with it). But they have to stay where they are, and end up having to watch it. +Some sort of aftermath where Steve obviously can't stay but he manages to talk to Bucky and tell him things will be okay. Bucky doesn't understand him or remember him.
I'd prefer if it were Stucky, either they used to have a relationship or pining after each other, but only friendship is okay too.
Trash party in the past
It can be as handwavey as you want, maybe they are trying to get HYDRA's information about the Tesseract, maybe the Asset is stationed somewhere with a team on a mission to kill Hank Pym/Peggy/Howard (perhaps he botched the mission so he is being pubished).
BONUS:
+ The person with Steve is Thor.
+They can't interfere for fear of fucking up the past (although that's what they'd be trying to do? I don't know how time travel will work in the MCU if they really go with it). But they have to stay where they are, and end up having to watch it.
+Some sort of aftermath where Steve obviously can't stay but he manages to talk to Bucky and tell him things will be okay. Bucky doesn't understand him or remember him.
I'd prefer if it were Stucky, either they used to have a relationship or pining after each other, but only friendship is okay too.