12 Best Fan Theories For 2018's Biggest Movies
10. The Super-Soldier Serum Is Derived From The Heart-Shaped Herb - Black Panther
Black Panther introduced audiences to Wakanda's naturally-occurring Heart-Shaped Herb, which would grant the ruler of Wakanda enhanced strength, speed and instincts, as well as the ability to cross over to the Ancestral Plane.
A Redditor has since suggested that the herb is in fact responsible for the synthesis of the Super-Soldier Serum, because Black Panther's (Chadwick Boseman) physical abilities are similar to those of a post-serum Steve Rogers (Chris Evans).
Furthermore, Cap's shield is made of vibranium and was created by Howard Stark, so we can assume that he had some sort of working arrangement with Wakanda, and may have been able to acquire (or more likely steal) some of the Heart-Shaped Herb to help complete the Super-Soldier Serum.
While it's probable this will never be officially confirmed in the MCU, it's still incredibly convincing.