We've got to presume now that in the first Cap movie, both Cap and Bucky will end up in the freezing ocean. This movie will also be set a little while after Cap has joined the Avengers.
Alright, here goes...
Captain America 2: The Winter Soldier
The film starts with a series of scenes, each one set in a different period of time after WW2. They all show assassinations and sabotage of American/Chinese/Japanese/German officers and military equipment...anyone the Russians have had any beef with. The assassinations are conducted by a shadowy figure. One or two scenes show glimpses of his costume, including his bionic arm. The time periods follow a general pattern of when Russia has been at war etc. One of these assassination is of Professor Zhang Chin (yeah I know he isn't killed yet in the comics but this is my movie).
Then we cut to the modern day and Cap. He's settled into the Avengers and is a massive national hero. Scenes reveal how, away from the limelight, he still feels guilt over Bucky's presumed death and he still mourns him.
Whilst Cap and Nick Fury are on the SHIELD Helicarrier when an unknown assassin attempts to kill Fury. Cap chases the unknown assassin but he escapes. Fury survives the assault but is sent to the medic bay. Here he informs Cap that the assassin is most likely the infamous Winter Soldier, a Soviet agent who has committed many murders in the past for the Russian military. Cap vows to stop him so Fury tells him to go with Black Widow (yay Scarlett Johannsen) to find him as she was once romantically involved with him when she too was an agent for the Russian military.
As they set off for Russia, she explains to Steve that the Winter Soldier was dragged out of the ocean during WW2. He had amnesia and was missing his left arm. (At this, Cap seems shocked for a moment as if thinking it could be Buck but then shrugs it off). She says that the Russians gave him a bionic arm and made him an agent, keeping him in stasis when he wasn't on a mission so that he wouldn't age as much.
I should probably say now, during the movie a Chinese man with a disfigured face (yeah this is the Man With No Face) is seen in a run down room. He is shown staring at a picture of Winter Soldier, whispering "I will kill you" (in Chinese with English subtitles. I like detail).
Cap and Black Widow then pursue clues and follow a path to find the Winter Soldier. Here they have various fights with Russian mobsters and henchmen etc.
They end up at the place where Professor Zhang Chin was killed and find the Man With No Face. Black Widow, aware of who he is, informs Steve that he is the bodyguard of Zhang Chin and is China's attempt at creating a Super Soldier. They confront him and he tells them that he vows to kill the person responsible for assassinating the man he was supposed to guard, making him a failure. He fights with Cap but manages to escape.
Winter Soldier, learning that he is being tracked by Cap and Black Widow, finds and confronts them. There is an awesome fight between Cap and Winter Soldier but Steve realises that it is indeed Bucky he is fighting. He tells Bucky to "Remember who you are." (Yeah, comic quote there for ya). Bucky regains his memories and is shocked to see Steve, as well as realise that he is an agent working for the Russians.
During this shindig of revelations, they are accosted by the Man With No Face who fights them. Steve, along with Bucky, defeat him. Bucky returns to the US with Cap and Black Widow but refuses rather than stay with Stever he says he needs to get away, find out who he is, and create a true identity for himself. Cue emotional goodbye between Steve and Bucky.
Back on the Helicarrier, Fury is originally annoyed that Steve didn't bring him in but is glad that the Winter Soldier is no longer active for Russia. Usual ending kind of scenes here, focusing on Cap.
End movie.
After credits scene. Set "sometime later" aboard the Helicarrier in Fury's office.
Fury's at his desk when the door opens but the person isn't revelaed (shot is focused on Fury).
Fury: "I see you got my message. You still want that job?"
Figure: "Yeah"
Fury: "Welcome to SHIELD"
End frame of Winter Soldier smiling.
Right then, I am aware that this may seem more like a Winter Soldier movie than a Cap one. I think that's just the way I wrote it. I do want however, for the main focus to be on Cap and his guilt over Bucky's "death". Also...leads into a possible Winter Soldier spin off film. Man, I'd love that!!!
Also, I apologise for hospitalising Nick Fury lol.
I do feel slighly guilty.
Finally, suggestions on who could play Winter Soldier.
I actually can't think of someone at the moment.