Chris Evans can’t play Captain America forever. Maybe he will get too old for the part, maybe his contract will run out, or maybe Marvel will inevitably kill Steve Rogers; the fact of the matter is Evans will one day step down from the mantle of Star Spangled Avenger. In the comics, both Falcon – played by Anthony Mackie in the MCU – and the Winter Solider – played by Sebastian Stan – have taken up the shield in the absence of Steve Rogers. Now, the Winter Soldier himself has some fairly strong opinions over who should become Captain America if Evans ever leaves.
During a Q&A session at Salt Lake Comic-Con, Sebastian Stan decisively answered the question:
Having Anthony Mackie become Captain America would be like Biff in Back to the Future II
The answer seems to indicate that Falcon taking up the mantle while the Winter Soldier exists in this universe would feel fundamentally unnatural. Stan’s quote refers to Back to the Future 2 where the villainous character Biff learns the secrets of time travel and uses it to turn protagonist Marty McFly’s life upside down.
From this it seems that, if Marvel decides to recast Captain America, that Stan believes Bucky Barnes is the logical choice due to the nature of his relationship with Steve. While Mackie’s Sam Wilson has been a loyal companion to Steve Rogers ever since they met in Captain America: The Winter Soldier, numerous seeds have been planted that allow Bucky becoming Cap to make much more sense – he has already wielded the shield twice. From his abilities to his backstory, Barnes represents a foil to Captain America – inexorably linked to his best friend. Allowing him to take over once Evans departs from the franchise could set Bucky up for a redemptive arc after spending decades on the wrong side of the fight working for Hydra.
By contrast, Falcon already has a fairly strong, independent persona that can exist outside of his relationship with Rogers. Beyond that, his appearance in Avengers: Age of Ultron and Ant-Man proved that the character has a strong enough presence within the universe to thrive without the possibility that he will ever pick up the vibranium shield.
As nothing has even been confirmed yet, this is all fan speculation -- although Stan's 9-picture deal with Marvel makes it seem incredibly possible. Stan mentioned in the video above, the power to make something like this happens lies in the hands of the fans. If Bucky Barnes ever does take over the title of Captain America from Steve Rogers, he will most certainly have some big shoes to fill.