Marvel green lit Captain America: Civil War as a result of WB first announcing Batman Vs. Superman. Now it's learned what the third Captain America movie was originally going to be about, which was going to see Cap vs. a zombie-like horde and not featuring Robert Downey Jr.'s Iron Man.
Joe and Anthony Russo reveal that Captain America 3 would have featured the Madbomb story from Captain America #193-200 by Jack Kirby, which sees Cap taking on an army of underground killers.
“There was a period where we did discuss a third act that revolved around the Madbomb from Cap mythology,” Anthony Russo told EW. “It didn’t have anything to do with Civil War, and if we couldn’t get Downey – in the very, very early conversations before we nailed him – somebody pitched the idea of a third-act that revolved around the Madbomb, which makes people crazy. It almost like zombifies them – but not literally.”
As noted, Captain America: Civil War couldn't have worked without Robert Downey Jr. The article notes Downey Jr. fullfilled his Marvel contract, so his involvment in further movies was up in the air; however, Marvel and Downey Jr. came to a new deal.
“The charm of the Madbomb is that you turn hordes of people into berserkers,” Anthony continued. “That was the physical challenge that Cap and company would have had to face.”
“The notion of the Madbomb would have been Cap having to fight civilians and how he would he handle that,” Joe Russo said. “We were always trying to put him into these interesting moral conundrums because of his nature. That would have made a compelling third act because if civilians are the antagonists, how could he stop them without killing them?”
“Somebody you know has turned into a zombie and now you have to fight them,” Anthony said. “And there would have been the emotional component of that.”