One of the most anticipated upcoming Disney+ Marvel shows is The Falcon and The Winter Soldier, which seemingly carries on directly from the end of Avengers: Endgame. That film's conclusion left a Captain America-sized hole in the Marvel Cinematic Universe, one which may not not be refilled the way that fans anticipate.

Recent casting confirms the presence of John Walker, a staunch supporting cast member of Avengers and Captain America comic books since the late 80s. With Steve Rogers off the table, John Walker's presence means that Captain America may return sooner than expected. This time, however, his version of the Star-Spangled Avenger could be the next big villain of the MCU.

RELATED: Russo Brothers Explain Why Winter Soldier Isn't Captain America

WHO IS JOHN WALKER?

John Walker first appeared in Captain America #323, by Mark Gruenwald and Paul Neary.  In that story, he was introduced as the "villain" Super-Patriot. After growing up in the South and idolizing his fallen soldier brother, Walker was a soldier who never saw the combat he yearned to fight in. Undergoing an experiment through the mysterious Power Broker, his faculties are upgraded to superhuman levels, beyond even Steve Rogers' super-soldier serum. Becoming a public figure, Super-Patriot criticizes Captain America's outdated variant of patriotism. His constant attempts to fight Captain America are typically rebuffed, though their one eventual fight ends in a tentative draw.

RELATED: How the MCU Failed Its Captain America Characters

Steve Rogers eventually gives up the mantle of Captain America after refusing to answer to the Commission on Superhuman Activities. The short search for replacement candidates passes over Falcon and Nick Fury before eventually landing on Walker, who is persuaded to become the new Captain America by Dr. Valerie Cooper.

This change was part of a larger trend of replacement Marvel heroes that also included new characters under the identities of Thor and Iron Man. The change was a relative success, and Walker remained Captain America for a year and a half. During this time, he begins to become more heroic, taking on the ideals of Steve Rogers. Despite this, he is also far more brutal and violent, using his superior strength to even kill enemies. Walker eventually relinquishes the title back to Steve Rogers, and after a huge personal loss, becomes U.S. Agent.

HOW BAD COULD JOHN WALKER BREAK?

US Agent Captain America

Not much is known yet about how The Falcon and The Winter Soldier will play out, or what role Walker will have in it. Given the loss of Steve Rogers as Captain America, however, the Marvel Cinematic Universe will be scrambling for a replacement to the icon. Since the comic book John Walker wanted to be an icon that replaced the outdated Steve Rogers, the MCU version may follow a similar path.

RELATED: Sam Wilson Not Taking Up the Captain America Moniker Is a Terrible Idea

If given the same origin of having a fallen military brother, MCU Walker may also resent Captain America for all of the attention that he takes from other soldiers. This would be a twisted variant of Steve's own origin, truly making Walker a dark reflection of him. This desire to be worshiped as his country's symbol may even be used by Zemo, who's returning in the series. All of these factors could make Walker's Captain America a villain to be taken down, not a paragon to look up to.

Sam Wilson Is Not Captain America

Captain America and US Agent fighting in #takebacktheshield

Anthony Mackie has recently stated that Sam Wilson will not take the Captain America mantle in the series. This ironically mirrors Walker's becoming Captain America in the comics. There, Sam Wilson turned down the offer, thinking that the country was not ready for a black Captain America. Winter Soldier may also be used in place of Nick Fury in the replacement short list in the MCU. With neither of his closest friends taking the mantle, the government, or some shadow organization, may choose Walker in desperation. This would also reflect early plot rumors involving the government not wanting Wilson to replace Rogers.

If Walker acts like his comic book counterpart, the plot of the show's first season writes itself. The violent, far more selfish Walker will run amok of the Captain America legacy. He may, however, eventually learn to more closely follow Steve's ideals. That could compel Falcon to try to change Walker's mind, forcibly take the mantle or have it pushed upon him by Walker the way it was on Steve in the comics. The former is more likely, and by defeating this false Captain America, Sam Wilson could prove that he was always the one worthy to wield the shield.

As the MCU enters uncharted territory, it needs one of Marvel's most iconic heroes, but as Phase 4 begins, the Marvel Cinematic Universe may deconstruct the legacy of the First Avenger, all before reconstructing it.

The Disney+ streaming service will launch this November. Directed by Kari Skogland, The Falcon and the Winter Soldier stars Anthony Mackie and Sebastian Stan and will be released on Disney+. The series will hit the streaming service in Fall 2020.

RELATED: Falcon and Winter Solider May Feature a Huge Link to Marvel's Netflix Shows