I know this sounds crazy but what if Bucky somehow gets his memories back and Falcon ends up getting his mind tampered with by the end of CA2? Sorta like a twist where Falcon becomes the villain and Bucky becomes a hero...Maybe Cap is able to get Bucky's mind back 2/3 of the way through the movie, so Zola gets mad and makes Falcon his next victim/subject...
In the comics Falcon once had his mind split by Red Skull, which created an alter ego called "Snap"...
"After an assignment in Rio de Janeiro, Sam's plane crashed in a remote Caribbean island where the Red Skull
and his henchmen, the Exiles
, were hiding out. The Red Skull sought to use Wilson as a pawn against Captain America
, who was currently on the island searching for the villain. The Skull reasoned that Wilson's idealism would appeal to the Captain enough to become his crimefighting partner. Then, at some later date, the Skull could use Wilson against his enemy. He used a Cosmic Cube
to revert "Snap" into Sam and give Sam the ability to telepathically communicate with birds, especially a bird that Wilson had bought named Redwing. Sam helped Captain America defeat the Skull and did indeed become his partner as the Falcon...Wilson's "Snap" identity remained submerged until some years later when he recalled his criminal past while undergoing shock therapy at S.H.I.E.L.D.
headquarters. Once his former criminal associates learned of this, a bounty was placed on his head. Wilson and Captain America fought off the attacks, however."
They could use some sort of mind programming techniques + hidden trigger words to cause Falcon to turn against Cap too, and the Avengers by the end...Joining him to the side of Ultron and the Masters/Hydra (who hide in the background throughout AoU). Falcon would end up fighting against War Machine and some of the others in Age of Ultron.
Crossbones doesn't need to be mind controlled into working for Hydra and leaving Shield. We would learn that Tony has been pulled into the programming, and doesn't know why his father really left (busy with Shield and Zola). Tony had parts of his memory/the IM3 story edited to trick his mind into believing parts of the stories. It was full of multiple cover stories. "Trevor" and Killian had to be present things in a tailor made way for Tony's brain, and are more like spies than soldiers--their brains think like spies. Parts of IM3 were just Tony telling a story (the memory he's recalling). It was edited before he tries to get Banner to listen up. All real, except the Tennessee parts, stories can be told improperly, exaggerated, missing pieces (from the 3hr+ cut) or misremembered. The Tennessee parts are implanted memories, and were altered to pre-condition him to be "the mechanic" and perceive things in certain ways. However, Iron Man is a part of his psyche and inseparable from him...Some of AoU would be like Inception. Inception-like battles between Stark and the real Mandarin. Through levels of inner space and cyberspace. His suit is attached to the image of his self concept within his mind. Would also be about freewill, Scarlett Witch and her mind powers, Ultron/Jarvis gaining freewill, and a piece of the mind gem Loki left on the scepter.
Rhodey has to face Falcon in an aerial battle and then a fully shape-shifted Killian in his true form.
A flying dragon shapeshifts out of the Killian brain/body through the tattoo.Rhodey finds out why Killian likes breathing fire: Killian's subconscious mind really reflects the image of the dragon. Killian/Foom only thinks it created Mandarin (and he did in some ways), but the dragon is a part of killian's mind, body, and psyche -- his true inner character once we pierce his outer shell. The Mandarin idea/name grows on Trevor, although Killian's mind took credit for it in IM3. Trevor brought "life to the character" that Killian/the Dragon created. The true Dragon form of Killian would reveal itself through innerspace and shapeshift through his body. It manipulated and attempted to control Trevor's brain through the powers of the subconsciousness and a mind link they shared; Trevor's not a decoy or double, "it's complicated", "there and not there", "made to do things", "panicked and handled it" (the dragon speaking through his mind). Trevor and Killian shared a secret mind link, each had a "live feed"..Coldblood did not know this, Killian could instantly control what Trevor said when Tony showed up if Trevor slipped into subconscious states to communicate with Killian telepathically, Killian told him to make Tony take the speedboat, all a manipulation... The dragon kept Trevor doped up to achieve subconscious/trance-like states and keep him under the influence. "Trevor" was a hydra superspy, uses multiple cover stories and false identities, this one has stuck with him. Shield realizes his name isn't in the registry right away. There's no Trevor Slattery by that desription, Shield has a problem, he won't give us his real name... So he gets thrown into maximum security until they figure things out. The only thing the prison staff can think to call him is The Mandarin, bringing that persona back to the forefront during a prison break.