In a show that's about the world's shifting relationship with the concept of Captain America and his legacy, it only seems totally fitting that it would also explore the Captain America: The First Avenger, the original scientist behind the serum, Abraham Erskine (Stanley Tucci), is assassinated by a HYDRA spy shortly after successfully transforming Steve Rogers. Erskine's version of the serum is destroyed, and the specific mix that created Captain America is forever lost, although this doesn't stop dozens of different organizations from trying once again.

In the comics, several world governments coalesced in the aftermath of World War 2 to try and replicate the super-soldier serum in several different capacities. These include Russian attempts such as the Red Guardian (David Harbour), splinter groups such as the Weapon Plus program (which created Wolverine and Fantomex), and finally, the U.S. military's own sinister experiments on black people such as Isaiah Bradley, a Black Captain America who secretly operated during World War 2 until the psychological experiences of his experiments left him with permanent brain damage. Carl Lumbly was recently cast in an undisclosed role in Falcon and Winter Soldier that many fans believe could indeed be Isaiah Bradley, which means the show is diving headfirst into the nefarious machinations of world governments in their attempts to create a super-soldier.

Related: The New MCU Timeline Breaks Agents Of SHIELD

Theory: Falcon and Winter Soldier Brings Back Abomination And The Leader

Despite The Incredible Hulk being considered generally canon within the MCU, there are a lot of loose plot threads that are never entirely cleaned up by the film or any other Hulk-centric movies in the franchise. General Thunderbolt Ross (William Hurt) turns up occasionally, but characters like his daughter Betty are written out of the series and referenced in a throwaway line. However, the biggest dangling idea left untouched by the rest of the MCU is the fate of the Abomination (Tim Roth), as well as Samuel Sterns (Tim Blake Nelson), who's villainous alter-ego The Leader is teased when drops of Bruce Banner's irradiated blood fall onto Stern's exposed head wound.

While these characters have been referenced in supplemental material, such as The Avengers one-shot prequel comic, fans haven't seen them return on-screen despite their massive storytelling potential. Part of it may be the complicated rights issues surrounding The Hulk as a character, but it's also entirely possible that Marvel wanted to save them for the right time and the right story. The Hulk's creation as a character has long been tied to secret attempts to recreate the super-soldier serum, and with the Abomination literally being a former United States mercenary, it would make perfect sense for him to show up in Falcon and Winter Soldier. As for Samuel Sterns, his introduction in The Incredible Hulk doesn't really imply a villainous nature to him, but that could very well have changed in the 12 years since the movie was released.

How Incredible Hulk's Characters Can Fit Into MCU Phase 4

Aside from providing some additional context on the super-soldier serum in the MCU, there's no reason why these characters can't also return in additional capacities within the MCU's next phase. Since Mark Ruffalo's Hulk is still set to appear every now and again in the series, audiences could finally see a rematch between The Hulk and the Abomination somewhere down the road, more than likely in Mark Ruffalo receives a role in the Disney+ MCU show. Similarly, The Leader has never been given the chance to properly work as a villain on-screen, so maybe Marvel could use his intellect and cunning as a mastermind villain for She-Hulk, making The Leader the architect of whatever events cause Jennifer Walters to gain Bruce Banner's abilities. There's a lot of options for these characters, and the storytelling potential has only continued to grow in the 12 years since the release of The Incredible Hulk, so audiences can only hope that The Falcon and Winter Soldier decides to use them in an exciting and fresh capacity.